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The protein-tyrosine kinase p56lck is the product of the lck gene. It plays a pivotal role in T-lymphocyte activation and thymocyte development, as indicated by the defective immune responses of lck−/− mice. We have demonstrated that an exon 7-deleted lck mRNA is produced by alternative splicing in all human cells expressing the lck gene. We have now looked for the protein encoded by this spliced lck mRNA and attempted to determine the function of the deleted Lck protein. This paper shows that the LckΔ7 protein is present in JCaM1.6 T-cells and we inferred that this isoform accounts for 15% of the total Lck proteins in the parental Jurkat T-cell line. We report that deletion of the first 51 amino-acids (exon 7) of the Lck catalytic domain greatly reduces the kinase activity of the recombinant protein. The residual activity can, nevertheless, be enhanced by adding Mn2+, whereas this cation has no effect on the activity of the p56lck mutated in its active site (K273E). The enforced production of LckΔ7 protein in transfected Jurkat cells results in slower cell proliferation than does p56lck. These findings suggest that the LckΔ7 protein is a p56lck cell-signaling regulator. This mechanism could be common to both humans and mice, in which we also found the exon 7-spliced lck transcript. [Copyright &y& Elsevier]