Ligation of the CD4 receptor by HIV envelope glycoprotein gp120 inhibits T cell activation and signaling through the TCR complex. Recent reports suggest CD4 ligation by gp120 + anti-gp120 Abs uncouples protein tyrosine kinases (PTKs) from the TCR signal-transduction cascade. This finding and other observations led us to hypothesize that the effects of gp120 are mediated through p56lck, a PTK noncovalently associated with CD4. To test this hypothesis, we first examined the kinetics of gp120/anti-gp120-induced TCR signaling defects in the Jurkat T cell line. Pretreating cells with gp120/anti-gp120 for 1 to 4 h before stimulation prevented TCR-directed PTK activation. Coincident with TCR desensitization, pretreatment with gp120/anti-gp120 also decreased the amount of p56lckthat could be immunoprecipitated from the Nonidet P-40 detergent-soluble fraction of cellular lysates, while simultaneously increasing the recovery of p56lckfrom the Nonidet P-40 detergent-insoluble fraction (or cytoskeleton). To assess the potential role of the actin in this process, experiments were conducted in the presence of cytochalasin D. Cytochalasin D restored TCR signaling in cells previously desensitized with gp120/anti-gp120 and prevented translocation of p56lckfrom the Nonidet P-40 detergent-soluble fraction of cell lysates. Furthermore, p56lckwas found to coimmunoprecipitate with anti-actin. These data suggest that gp120/anti-gp120 may inhibit TCR signaling by sequestering p56lckto the cytoskeleton.