Evidence is presented which suggests that after a period of adsorption of 30 min. the herpes virus which could not be removed from HeLa cells by repeated washing existed in 4 fractions: Fraction A, about 63%, which became noninfective and insusceptible to antiserum over the next 2 hr.; fraction B, about 16%, which remained susceptible to antiserum and infective for up to 6 hr. after adsorption; fraction C, about 2%, which remained infective and insusceptible to antiserum for at least 6 hr. after adsorption; fraction D, about 19%, which became noninfective and insusceptible to antiserum during the adsorption period. Replication occurred from fractions A and D. Inactivation and the development of insusceptibility to antiserum occurred very close together in time, possibly simultaneously. Incubation of infected cells in the presence of antiserum lowered the yield of virus obtained after 48 hr. incubation; this could not be explained simply by prevention of secondary infection through the medium.