A human CD4+ T cell line, Jurkat, was transfected with a constructed plasmid, which has the envelope gene of the human immunodeficiency virus (HIV) under the transcriptional control of the human metallothionein IIA promoter, and these transfected cells were then cloned. JME2, one of the cloned cell lines, expressing the envelope glycoprotein after induction with metal ions, showed the ability to form syncytia involving other CD4+ cells not expressing the HIV envelope protein. When several CD4+ cell lines were examined for their susceptibility to syncytium formation by JME2 cells, the p56lck-expressing cell lines were found to be more susceptible to syncytium formation than the p56lck-non-expressing cell lines. To substantiate the role of p56lck in the syncytium formation, a CD4+, p56lck-non-expressing monocytoid cell line, U937 clone 2, was transfected with an lck-expressing construct. Using such transfectant cell clones, it was demonstrated that p56lck-positive cells are markedly more susceptible to the syncytium formation than p56lck-negative cells, implying a regulatory role for p56lck in syncytium formation mediated by the HIV envelope and CD4 molecule. Moreover, it was suggested, in the experiments using CD45 cross-linking or a protein tyrosine kinase inhibitor, genistein, that p56lck affects syncytium formation through its protein tyrosine kinase activity. A putative mechanism by which p56lck affects the syncytium formation is also discussed.