Platelet-derived growth factor (PDGF) regulates clonal proliferation of malignant pre-B cell lines, but little is known about its role in normal B lymphocyte differentiation and malignant transformation. To understand the expression of PDGF-A, PDGF-B and the β-receptor (PDGF-Rβ) in B cell lymphoproliferative disorders, we used an immunohistochemical method to stain formalin-fixed, paraffin-embedded tissues in 5 patients with reactive lymphoid hyperplasia, 15 with non-Hodgkin’s lymphoma and 23 with B cell chronic lymphocytic leukemia (B-CLL). Abundant PDGF-A, rather than PDGF-B, was expressed in normal B cell differentiation. There was no difference in the expression of PDGF-A and PDGF-B between patients with reactive lymphoid hyperplasia and patients with malignant lymphoproliferative disorders. Among the patients with B-CLL, the expression of PDGF-B was much stronger than the expression of PDGF-A, and 18 of the patients had coexpression of PDGF-B and PDGF-Rβ. A larger proportion of patients with B-CLL than with non-Hodgkin’s lymphoma had expression of PDGF-B and PDGF-Rβ. In conclusion, PDGF-A expression in all stages of B lymphocyte differentiation suggests that it is important in B cell differentiation and proliferation. Expression of PDGF-B and PDGF-Rβ suggests that autocrine signaling of PDGF may be important in malignant transformation of B-CLL. However, further studies are necessary to confirm these conclusions.