Triggering of the multicomponent T cell antigen receptor (TCR) complex results in several biochemical processes which are critical for the functional activation of T lymphocytes. One common process is the tyrosine phosphorylation of several proteins, including the TCR zeta chain. Here we show that in addition to TCR zeta, other subunits (CD3 gamma, CD3 delta, and CD3 epsilon) of the TCR complex can also be tyrosine-phosphorylated in response to antigen receptor stimulation. This rapid phosphorylation was detected in several mature murine T cell subsets, including CD4+ type 1 and 2 helper cells (TH1 and TH2). Therefore, tyrosine phosphorylation of multiple TCR components in addition to TCR zeta may be an important event during the initiation of the signaling cascade leading to T cell activation.