The cyclostomes, hagfishes and lamprey, represent modern agnathans, the most primitive vertebrates. They are therefore of special interest from the phylogenetic view point with regard to proliferative activities of epithelial and of lympho-hematopoietic tissues. The animals, kept in aquaria at 15 C, were given 1.0 μCi of 3H-thymidine per gram of body weight intramuscularly, killed 2 hr later, different organs prepared for autoradiograms using the liquid emulsion technique, and the labeling indices determined. In peripheral blood, cell proliferation occurred mainly in the hemocytoblast group of cells in both species. Both lympho-hematopoietic cells and epithelial cells proliferated in the lamprey, although at a relatively low rate, perhaps attributable to senescence. In the hagfish, blood-forming and epithelial cells were rapidly proliferating, with the dramatic exception of intestinal epithelium, where the proliferative activity was extremely low. This Finding may well explain the documented high resistance of hagfishes to irradiation and alkylating agents, in contrast to the lamprey, which is approximately as sensitive to these agents as most advanced vertebrates.