The effect of 3'-azido-3'-deoxythymidine (AZT) on replication of human immunodeficiency virus (HIV) in various hematopoietic cell lines was investigated. The concentration of AZT required to block HIV replication varied depending on the cell line used. U-937 cells required as little as 0.01 microM AZT to block HIV replication, a concentration almost 100 times lower than that required for MT-4 or MOLT-4 cells. However, 467 and TALL-1 cells required concentrations of AZT higher than 5 microM. It was clear that AZT was ineffective once the viral gene was integrated into chromosomal DNA; removal of AZT from culture fluids at that stage allowed the full expression of HIV. The effect of the potent immunostimulator lentinan was also examined, and it was shown that lentinan enhanced the effect of AZT.