In 112 untreated myeloma patients we have analysed the immunophenotype of plasma cells both by immunofluorescence (IF) and immunocytochemistry (APAAP). Both techniques yielded similar results pointing to an important degree of heterogeneity in antigenic expression not only between different patients but also within the same patient. The expression of CD38 and Han-PC1 antigens (Ags) was almost constant (greater than 90% positive cases), while CD9 was detected in 66% of the cases. On the other hand, less than one third of patients were positive for CD10, CD20 and HLA-DR and generally with a weak expression (less than 30% positive plasma cells). In occasional cases plasma cells were weakly positive for the myelomonocytic markers CD13 (9%), CD15 (25%) and CD14 (6%). The possibility that this heterogeneity might be the result of different stages of differentiation of the neoplastic clone is suggested both by the positive correlation in the expression of some of these antigens (CD10, CD9, CD20, HLA-DR) and by the relationship between CD10 and myeloid antigens with immature plasma cell morphology. Finally, the cALLA antigen does not seem to be of significant value in predicting survival. Moreover, none of the other markers explored showed a clear influence in the course of the disease, although the tendency towards a lower survival found for the CD20+ cases as well as the association of the expression of some antigens and advanced clinical stage, may warrant further studies in a larger series of patients.