Signaling through the T cell antigen receptor (TCR) results both in rapid increases in tyrosine phosphorylation on a number of proteins and in the activation of the phosphatidylinositol pathway. It is not clear how stimulation of the TCR leads to these signaling events. Mutants of the Jurkat T cell line have been previously isolated that fail to show increases in calcium following receptor stimulation. Analysis of one of these mutants, JCaMl, which is defective in the induction of tyrosine phosphorylation, revealed a defect in the expression of functional Ick tyrosine kinase. The lack of Ickactivity was caused in part by a splicing defect. Expression of the/c/r cDNA in JCaMl restores the ability of the cell to respond to TCR stimulation. These results indicate that Ick is required for normal signal transduction through the TCR.