The MRC OX52 monoclonal antibody is a marker of rat T lymphocytes. We have cloned by polymerase chain reaction the rat homologue of CD6, and fluorescein-activated cell sorter analysis and immunoprecipitations using OX52 in COS7 cells transfected with rat CD6 cDNA showed that CD6 is the cell-surface molecule recognized by OX52. Immunoprecipitation analysis showed that CD6 coprecipitated with CD5, which in turn, was coprecipitated equivalently with CD2, CD6, and the T cell receptor (TCR), but the fraction of CD5 associated with CD6 was highly phosphorylated in kinase assays, in marked contrast with the low level of phosphorylation of CD5 associated with TCR or CD2. Examination of protein kinases associating with these antigens showed that paradoxically, CD2 coprecipitated the highest amount of Lck and Fyn. CD6 also associated with Lck, Fyn, and ZAP-70, although at lower levels but additionally coprecipitated the Tec family kinase Itk, which is absent from CD2, CD5, and TCR complexes. Lck together with Itk was the best combination of kinases, effectively phosphorylating synthetic peptides corresponding to a cytoplasmic sequence of CD5. Overall, our results suggest that CD6 has an important role in the regulation of CD5 tyrosine phosphorylation, probably as a result of its unique feature of associating with kinases of different families.