The CD4 T-cell surface antigen and the lymphocyte-specific tyrosine-protein kinase p56lck form a stable noncovalent complex in CD4+ T-lymphocytes. In this report, we demonstrate that these two gene products can also associate when co-expressed in NIH3T3 fibroblasts, therefore implying that other lymphoid specific components are not required for the CD4-lck interaction. These results also suggest that co-expression of CD4 and p56lck in non-lymphoid cells may prove to be a useful model system for the analysis of structural and possibly functional CD4-lck interactions.