We have succeeded in growing the HTLV-IIIB strain of human immunodeficiency virus type 1 (HIV-1) in cultured human thymic epithelial (TE) cells. Expression of the HIV-1 proteins p17 and p24 was detected by immunofluorescence and reached a peak at 3 days after infection. Antigen capture and reverse transcriptase assays were used to detect HIV-1 in culture fluids, with positive results also being realized. The infection was cytolytic; cellular disarrangement, increased numbers of Hassall's corpuscles, and giant cells first appeared in monolayers of TE cells at 2 days after inoculation. By 4 days these changes were increased, and by 7 days, retraction and involution of TE cells were evident. The infection of TE cells by HIV-1 was blocked by preincubation with monoclonal OKT4A antibodies directed against CD4 target molecules.