Clustering of beta 1-integrins on adherent cells with antibodies or ligands results in increased tyrosine phosphorylation and activation of a novel focal adhesion tyrosine kinase, pp125FAK. The genes encoding pp125FAK have been cloned previously from both chicken and mouse cDNA libraries, and the deduced amino acid sequences are nearly identical (94%). Two synthetic peptides derived from sequences at the carboxyl terminus of chicken pp125FAK were conjugated to ovalbumin to generate rabbit heteroantisera. Human pp125FAK was immunodetected in both T and B lymphocytes with these antisera. A basal state of pp125FAK tyrosine phosphorylation was observed in T and B lymphocytes, and its expression level was in general augmented among human T- and B-cell leukemia/lymphoma lines. Additionally, the full-length sequence of human T-cell pp125FAK (huT-FAK) was derived from a Jurkat T-cell cDNA library. huT-FAK is structurally identical with both mouse and chicken FAK, and shares 95% amino acid identity with chicken pp125FAK and has 97% homology with the mouse sequence. This high degree of evolutionary conservation between species suggests that pp125FAK is likely to have a crucial function in the cell. Expression of the full-length huT-FAK gene in COS cells showed an immunologically indistinct human pp125FAK protein compared with the endogenous primate pp125FAK. Taken together, the data indicate that this structurally conserved human T-cell pp125FAK likely functions in T- and B-cell lineages, and its altered expression in human lymphocyte tumor cell lines may contribute to their transformed phenotype.