Treatment of HUT78 cells with CD4-binding peptide constructs derived from the C4 domain of HIV-1 gp120 results in autophosphorylation of a src-related kinase, p56lck. This leads to p56lck activation and the subsequent phosphorylation of tyrosine residues in several intracellular proteins. The phosphorylation is specific to the C4 peptides as no new phosphorylation occurs when the cells are treated with control peptides or polymers. The induction of tyrosine phosphorylation by the C4 peptide constructs depends on the capability of the peptide to assume a helical conformation because similar peptide constructs that were not able to form helices did not induce cellular tyrosine phosphorylation.