Unlike other B cells, plasma cells (PC) react with only a few antibodies against haemopoietic antigens. We investigated 36 specimens exhibiting a reactive increase in PC numbers (i.e. plasmacytosis, PC hyperplasia) with a broad panel of antibodies suitable for use on formalin-fixed, paraffin-embedded tissue, and compared the findings with those obtained in 51 cases of multiple myeloma (plasmacytoma). Regardless of the immunostaining pattern for immunoglobulin light and heavy chains, reactive PC reacted with at least two and at most six of seventeen antibodies detecting haemopoietic antigens [Ber-H2/CD30 (91%), anti-leucocyte common antigen (LCA)/CD45 (86%), KP1/CD68 (64%), MB2 (57%), 4KB5/CD45RA (37%), DF-T1/CD43 (28%), UCHL1/CD45RO (20%), L26/CD20 (17%), MT2 (14%) and Mac387 (8%)], and with at least one and at most four of six antibodies against non-haemopoietic antigens [anti-epithelial membrane antigen (EMA) (94%), anti-vimentin (77%), anti-pan-cytokeratin/KL1 (74%), BMA120 (51%) and HMB45 (14%)]. Five antibodies stained reactive PC significantly more often than neoplastic PC: Ber-H2/CD30 (p < or = 0.0000), KP1/CD68 (p < or = 0.0000), anti-LCA/CD45 (p < or = 0.0000), anti-EMA (p < or = 0.0339) and anti-pancytokeratin/KL1 (p < or = 0.0000). The more frequent and more heterogeneous expression of antigens by reactive PC suggests that the aberrant immunoreactivity of neoplastic PC in plasmacytoma is not due to the process of malignant transformation in an early step of B-cell differentiation, but could reflect the heterogeneity of antigen expression by normal PC.