Src-family nonreceptor protein tyrosine kinases (NRPTK) are associated with cell surface receptors in large detergent-resistant complexes: in epithelial cells, yes is selectively located in vesicle structures containing caveolin (“caveolae”). These formations are typically also endowed with glycophosphatidylinositol (GPI)-anchored proteins. In the present study, we observed lck, lyn, src, hck, CD4, CD45, G proteins, and CD55 (decay-accelerating factor) expression in the buoyant low- density Triton-insoluble (LDTI) fraction of selected leukemic cell lines and granulocytes. We provide a detailed analysis of the two most highly expressed NRPTK, p53/p56lyn and p56lck, which are involved in the transduction of signals for proliferation and differentiation of monocytes/B lymphocytes and T lymphocytes, respectively. We show that lyn is selectively recovered in LDTI complexes isolated from human leukemic cell lines (promyelocytic [HL-60], erythroid [K562] and B- lymphoid ) and from normal human granulocytes, and that lck is recovered from LDTI fractions of leukemic T- and B-lymphoid cell lines (CEM, 697). In LDTI fractions of leukemic cells, lck and lyn are enriched 100-fold as compared with the total cell lysates. Analysis of these fractions by electron microscopy shows the presence of 70- to 200- nm vesicles: lyn and lck are homogenously distributed in the vesicles, as revealed by an immunogold labeling procedure. These novel results propose a role for these vesicles in signal transduction mechanisms of normal and neoplastic hematopoietic cells. In support of this hypothesis, we further observed that molecules participating in B- and T-cell receptor activation cofractionate in the LDTI fractions, CD45/lyn (B cells) and CD45/lck/CD4 (T cells).