The tyrosine protein kinase p56lck, specifically expressed in lymphoid cells, undergoes modifications of its autophosphorylation and kinase activity when these cells are triggered by mAbs to the T cell determinants. The kinase activity and the autophosphorylation of p56lck were analysed following triggering Jurkat cells with the human immunodeficiency virus (HIV) glycoprotein gp160 which interacts with CD4: both the autophosphorylation and the kinase activity are increased within 1-5 min following addition of gp160, this increase is maximum at 5 min and is followed by a gradual return to the basal level within 2 h. Similar to observations made with anti-CD4 mAbs the increase in kinase activity of p56lck is not associated with changes in the gel mobility nor is it associated with T cell activation. Triggering of T cells with a combination of anti-CD3 mAbs which activate T cells but not p56lck and gp160 greatly potentiated the increase of p56lck autophosphorylation and kinase activity.