In 16 patients with monoclonal gammopathies of undetermined significance (MGUS) and in 49 with multiple myeloma (MM, 43 untreated and 6 relapsed) we used immunocytochemistry to determine the percentages of bone marrow plasma cells (BMPC) that incorporate bromodeoxyuridine (BUDR-labeling index, BUDR-LI) in vitro and that label with the monoclonal antibody Ki-67 (which recognizes an antigen thought to identify the growth fraction of the population, Ki-67 GF). Both mean and range values were greater for Ki-67 GF than for BUDR-LI. Most patients with high Ki-67 GF also had high BUDR-LI, although a linear correlation was not found between the two parameters. MGUS has lower values than MM, and the difference was much greater for Ki-67 GF than for BUDR-LI (p less than 0.005 vs. p less than 0.05). Differences in Ki-67 GF but not in BUDR-LI were found between MGUS and stage I MM (p less than 0.0005) and between grouped stage I and II MM and stage III MM (p less than 0.025). Both Ki-67 GF and BUDR-LI were significantly (p less than 0.005) greater in relapsed than in untreated MM. Determining Ki-67 GF as a proliferative parameter could be a better way of studying the kinetics of (BMPC) in MGUS and MM than determining the BUDR-LI, since a wider range of values is obtained and this allows patient groups with different clinical characteristics to be separated more easily.