The results of an investigation of the ability of a strain of influenza A virus and of herpes simplex virus to propagate in fibroblasts cultured from different tissues of the chick embryo have been presented. The data indicate that influenza virus, possessing rather narrow tissue trophisms, does not grow in fibroblasts derived from susceptible or nonsusceptible tissues of the chick embryo. Herpes virus, which has a broader affinity for different cell types, was readily cultivated and produced cellular cytopathogenicity in similar cultures. Influenza virus failed to multiply in fibroblasts from the lung, heart, skeletal muscle, or cornea tissue, although ability of this virus to grow in respiratory cells of the embryo itself has been reported (11). The same virus seemed to propagate in primary cultures of lung and cornea, presumably in susceptible cells carried over from the animal host. The growth of the virus in primary cultures of lung and cornea but not in skeletal muscle or heart agrees with results of Pearson and Enders (12). Although influenza virus appeared to grow in the primary explants of lung and cornea, the virus did not seem to produce cytopathogenic changes in the fibroblast outgrowth of these tissues Fibroblasts derived from lung tissue supported the growth of herpes simplex virus in plasma clot cultures; evidence for its propagation in fibroblasts from skeletal muscle was less striking. Cellophane cultures of both lung cells and heart fibroblasts produced unequivocal growth of the virus. These observations conform with the broad affinity of herpes virus for various cell types in the chick embryo described by Anderson (13) who found herpes inclusion bodies in cardiac muscle, in the interstitial cells of the lung, in cells of striated muscle and in other mesodermal elements. Similar findings for this strain of herpes virus and various cell types in newborn infants were described in an earlier paper from this laboratory (14). In contrast to influenza virus, not only does herpes virus multiply in fibroblasts, but with viral propagation destruction of the cells occurs until few normal cells remain and virus production ceases. Similar observations have been made by Bang and Gey (3), using Eastern equine encephalomyelitis virus in a cultured line of tumor cells. A direct cytopathogenic effect of herpes virus on the cells of plasma clot cultures was not evident, although in one experiment subcultures of cells from the infected cultures did not grow as well as uninfected control cultures. It seems, therefore, that cytopathogenic effects may be demonstrated more clearly in non-plasmatic cultures. This observation also has been made by Gey and Bang (2) who showed that Newcastle disease virus in plasma clot cultures of chick muscle fibroblasts may not be able to reach and destroy isolated strands of cells buried in the clot. The cytopathogenic effect of poliomyelitis viruses on the fibroblastic outgrowth of human embryonic tissues was demonstrated by Robbins and coworkers (15). Other workers (16, 17) have confirmed these findings and have demonstrated that strains of poliomyelitis viruses show differences in cellular pathogenicity for the flbroblast outgrowths. Since growth of the viruses in fibroblasts alone was not reported, the question remains whether the viruses were growing in the explanted tissues and exerting a toxic effect on the fibroblast outgrowth or whether degeneration of the fibroblasts resulted from the viruses growing within them. Either or both of two possibilities may account for the failure of influenza virus and the ability of herpes virus to multiply in fibroblast cultures. First, the fibroblasts may have been derived from connective tissue elements which were or were not susceptible to these viruses in the animal host. Second, the fibroblasts may have originated from susceptible cells in vivo but lost their ability to support the propagation of influenza virus after prolonged culture and transfers in vitro. The solution of this problem depends on whether fibroblasts derived from different tissues have varying inherent characteristics or whether they represent a common cell prototype in the animal host and subsequently develop individual characteristics in vitro (6).