Herpes simplex is among the most ubiquitous of viruses capable of infecting man, and the clinical manifestations of the infection it induces are various. Until recently no simple and rapid serologic test has been available by which the antibody response to infection with this virus could be determined. When serologic studies have been carried out, the virus neutralization test usually has been employed, although antigens for use in complement fixation and skin tests have been described. The present study was undertaken to explore the value as antigens in the complement fixation test of amniotic and allantoic fluid from infected chick embryos. When such materials were shown to be effective in this respect, the reliability of the test as a diagnostic procedure and as a tool for the epidemiological study of herpes infections was investigated. Earlier workers who applied complement fixation tests to the assay of herpes antibody used antigens prepared