We studied the effect of AZT on the replication of HIV-1 in freshly and chronically infected U-937 monocytoid cells over a period of 2 months. Expression of viral antigens was monitored by indirect immunofluorescence, and viral replication was assessed by reverse transcriptase assay on virus pelleted from culture fluids. In U-937 cells not treated by AZT, viral antigens were expressed by 7 days after infection. The inclusion of a variety of concentrations of AZT in the culture medium was shown to retard virus replication in a dose-dependent fashion, although a complete inhibitory effect was not seen with any clinically attainable concentration of drug. Exposure of HIV-1-inoculated cells to AZT did not give rise to progeny virus possessing a drug-resistant phenotype. However, the study of clonal derivatives of U-937 cells revealed cellular variants with increased susceptibility to HIV-1, and against which AZT had reduced effectiveness in comparison with the parental line. No effect of AZT was seen on cells already infected by HIV-1, suggesting that this drug had no influence on viral replication in U-937 cells into which viral DNA had previously integrated.