The role played by the cytoplasmic domain of the CD4 molecule in the process of HIV infection was investigated, using A2.01 cells which express different forms of the CD4 gene. A delay in HIV production was consistently observed in cells expressing a truncated CD4 which lacks the cytoplasmic domain (CD4.401) compared with cells expressing the wild type CD4. The delay was much less in cells expressing a hybrid CD4‐CD8 molecule (amino acids 1‐177 of CD4 fused to the hinge, transmembrane and cytoplasmic domains of CD8). Yet the extent of viral entry and reverse transcription, monitored by semi‐quantitative PCR, was similar in each cell type studied. For further study of the mechanism responsible for delayed HIV replication in the A2.01/CD4.401 cell line, cells were treated with phytohaemagglutinin (PHA), 24 h after HIV infection. Under such experimental conditions HIV production was detected at the same time in the culture supernatants of A2.01/CD4 and A2.01/CD4.401 cells. Moreover, we found that CD4 oligomerization by HIV‐1 induced NF‐kappa B translocation in A2.01/CD4 and A2.01/CD4‐CD8 but not in A2.01/CD4.401 cells. This was consistent with CAT assay experiments which provided evidence for Tat‐independent NF‐kappa B mediated activation of HIV‐1 LTR promoter after HIV binding to CD4 in A2.01/CD4 and A2.01/CD4‐CD8 but not in A2.01/CD4.401 cells. In contrast to results published recently by Tremblay et al. (1994, EMBO J., 13, 774‐783), we propose that a positive cellular signal initiated following oligomerization of the CD4 by the virus itself is involved in NF‐kappa B‐dependent early HIV transcription in A2.01/CD4 cells.