Monocyte-enriched cultures of human blood leukocytes were exposed to herpes simplex virus (HSV) at different multiplicities of infection (m.o.i., from about 10 to 0.001 p.f.u./cell). Highest maximum progeny virus titres were invariably obtained with low initial m.o.i., i.e. those between 0.01 and 0.0001 p.f.u./cell, while little if any infectious progeny was produced in cultures inoculated with the highest virus concentrations. By the time of maximum virus production, i.e. 5 to 7 days after inoculation, monocytes in the uninfected cultures had mostly differentiated to macrophages. This differentiation was partially inhibited in cultures initially exposed to the higher concentrations of HSV. Synthesis of HSV antigens was detected by indirect immunofluorescence both in the high m.o.i. cultures and in the productively infected cultures. By this criterion, a maximum of 10 to 15% of all adherent cells became infected in both culture types. It is suggested that the higher doses of HSV, by inhibiting cellular maturation, also prevent the subsequent completion of its own infectious cycle.