Human immunodeficiency virus 1 (HIV-1) produced in the human T lymphoblastoid H9 cell line infected cells of that line more readily than cells of the human monocytoid U937 line. While both cell lines expressed detectable levels of the CD4 molecule on their surfaces, the H9 and U937 cell lines differed in expression of major histocompatibility complex class I and class II antigens. Both H9 and U937 cells were infected initially with HIV-1 derived from H9 cells. Cell-free culture supernatants were harvested after the cells had been infected for at least 1 month. Culture supernatant from HIV-infected H9 cells was used to infect H9 and U937 cells. Conversely, culture supernatant from HIV-infected U937 cells was used to infect H9 and U937 cells. The percentages of cells infected at each of several time points during the first few days after infection were determined by flow cytometric analysis of cell-associated HIV-1 major core protein p24. Infection of each cell line was more efficient when the cell type infected was identical to that in which the infecting supernatant was produced. However, this difference in tropism was not generated early after infection of each cell line, as might have been expected if this effect were mediated by cell surface molecules acquired during the process of budding through the cell membrane.