The monoclonal antibody Ki-67 reacts with a nuclear antigen that is present only in proliferating cells. The proportion of Ki-67 positive cells may therefore serve as a reliable measurement for the growth fraction in normal and neoplasmic cell populations. We have tested the significance of the MoAb Ki-67 in the classification of monoclonal gammopathy and compared the results with the plasma cell labelling index. In benign monoclonal gammopathy the percentage of Ki-67 positive plasma cells (median 1.6%) was significantly lower than in untreated multiple myeloma (median 9.6). Among the patients with more than 10% Ki-67 positive plasma cells there were some very short survivors. The largest growth fractions (median 41.8%) were found in patients with relapsing multiple myeloma indicating here a different growth pattern more resembling the high-grade lymphomas. A linear correlation between the proportion of Ki-67 positive plasma cells and the labelling index was not found. Determination of the plasma cell growth fraction with the monoclonal antibody Ki-67 in monoclonal gammopathy may help to discriminate benign monoclonal gammopathy from multiple myeloma and will probably identify a subgroup of multiple myeloma patients with a poor prognosis, including those with relapsing multiple myeloma.