The CD45 antigen is a family of the transmembrane tyrosine phosphatases expressed on cells of haemopoietic origin. The molecules are distinguished by the different aminoacid sequence and glycosylation on the N terminus. Although all isoforms are heavily glycosylated and exert receptor like structures on the extracellular part, the role of the glycosylation in the possible receptor function and the ligand of the CD45 has not been determined yet. In this study we have examined the binding of phytohaemagglutinin (PHA) to the different isoforms and its relation to the phosphatase activity. Immunoblotting experiments revealed that the 220 kDa form of the CD45RA contained a PHA binding fraction when immunoprecipitated with CD45RA monoclonal antibody (mAb), while an isoform with identical molecular mass immunoprecipitated by anti-CD45 did not bind PHA. We concluded that the 220 kDa form was heterogeneous with respect to PHA binding. Functional data also confirmed this heterogeneity: previous extraction of the PHA binding proteins resulted in the elimination of all the phosphatase activity from CD45, while only a part of that was removed from CD45RA immunoprecipitates.