Early in HIV infection, CD4+ lymphocytes exhibit the properties of an anergic state characterized by unresponsiveness to mitogens or to TCR stimulation and by defective IL-2 production. As tyrosine phosphorylation is the earliest of the biochemical events initiated by stimulation of CD3-TCR, we studied protein tyrosine phosphorylation in purified CD4+ lymphocytes from 25 asymptomatic seropositive patients (CD4 T cells > 350/mm3) previously stimulated in vitro by immobilized anti-CD3 mAb or by co-immobilized anti-CD3 and anti-CD28 mAbs. Purified CD4+ lymphocytes from HIV-infected patients exhibited defective early protein tyrosine phosphorylation in response to CD3 activation when compared with normal subjects. This defect was observed mainly in patients in whom proliferative responses to immobilized anti-CD3 ranged from 2 to 50% of control values obtained in healthy donors, and was frequently associated with increased cellular levels of p59fyn and decreased cellular levels of p56lck. Interestingly, these defects appeared to correlate with the degree of impairment in thymidine incorporation. Since CD28 mAbs have been reported to enhance proliferative responses to the CD3-TCR pathway in cloned murine or human anergic models and to induce tyrosine phosphorylation in human T cells, we studied the role of CD28 mAb as a co-signal. Although anti-CD28 co-stimulation augmented the proliferative responses in both controls and HIV-infected patients, it failed to correct the tyrosine phosphorylation pattern in the latter. Our results suggest a relationship between defective early protein tyrosine phosphorylation and impairment of proliferative responses in CD4 T cells from HIV-infected patients, and evidence is provided that associated altered cellular levels of the fyn and lck tyrosine kinases might play an important role in the anergic response observed early during HIV infection.