Lymphocytes were separated from peripheral blood of adult human donors by Ficoll-Hypaque gradient centrifugation and cultured in the presence of nonspecific mitogens (phytohemagglutinin[PHA] and concanavalin A) or specific microbial anatigens (herpes simplex virus [HSV], mumps virus, streptococcal enzymes, and Candida albicans). Exposure of lymphocyte cultures to infectious HSV resulted in almost complete inhibition of blastogenesis ([3/H]thymidine uptake) induced by each of the mitogens and antigens, a finding which suggests that a common mechanism may underlie the inhibitory effect. Several characteristics of the effect of virus on blastogenesis were noted: (1) virus inactivated by heat or ultraviolet irradiation was ineffective; (2) inhibition (is greater than 90%) was greatest in cultures exposed to HSV on or before the addition of PHA; (3) lymphocyte preparations washed free of HSV continued to be refractory to stimulation, an observation indicating that the presence of unabsorbed virions or viral products was not essential; and (4) inhibition was independent of the cell donor's state of humoral immunity to HSV.