Cytomegalovirus (CMV)-induced stimulation of cellular DNA synthesis was studied by autoradiography-immunofluorescence in susceptible human embryonic lung (HEL) cells in which DNA synthesis had been suppressed by serum starvation or treatment with 5-fluorouracil. Those cells in the culture which had been stimulated to synthesize detectable amounts of cellular DNA between 24 and 48 hr did not synthesize detectable amounts of viral antigens even as late as 96 hr postinfection, indicating that infection was abortive in these cells. To determine the role of defective CMV particles in the stimulation of cellular DNA synthesis, the ability of populations of virions obtained after undiluted serial passage to induce host cell DNA synthesis was compared to that of populations of recently plaque-purified virions. Stocks obtained after amplification at low multiplicity of three individual plaques were all poor inducers of cellular DNA synthesis. On the other hand, undiluted, serially passaged CMV stocks were very effective in stimulating cellular DNA synthesis. After low doses of uv irradiation the ability of standard virus stocks to induce host cell DNA synthesis also increased; stimulation of cellular DNA was apparent even at uv doses which eliminated by more than 99.5% the ability of the virus to induce the synthesis of viral antigens. We conclude that CMV particles that are incapable of giving rise to the synthesis of detectable amounts of either early or late viral antigens and that are presumably defective are present in standard CMV stocks and play an important role in the stimulation of cellular DNA synthesis in susceptible cells.