To better define the role of interleukin-3 (IL-3) and IL-6 in the cytogenetic analysis of multiple myeloma (MM), we performed concomitant chromosome and cytologic studies in 34 patients. In each case, 10-30 x 10(6) bone marrow cells were incubated in two independent cultures consisting of conventional cytogenetic medium with and without IL-3 plus IL-6 added for 72 hours. 1-ml aliquots of each culture were aspirated at 24, 48, and 72 hours and exposed to colcemid for 6 hours. Cytospin preparations were then made and mitotic cells were counted and identified as plasma cells or as nonmalignant cells based on their reactivity with an appropriate anti kappa/lambda serum. Slides for conventional cytogenetic analysis were prepared at 72 hours. A greater than two-fold increase of mitotic plasma cells was observed in cytospin preparations from stimulated cultures versus unstimulated cultures in 15 of 34 cases, whereas a less than 2-fold increase, no variation or no mitosis was recorded in 19 cases. Comparison of the number of mitotic plasma cells in stimulated cultures at 24, 48, and 72 hours showed a decreased mitotic activity at 72 hours. Clonal abnormalities were detected by conventional cytogenetic analysis in 19 of 34 cases (55.8%). Recurrent clonal aberrations involved chromosome 13 (4 cases), chromosomes 1p, and 14q (3 cases); chromosomes 3p, 6q, 7q, and 9q (2 cases). We conclude that IL-3 + IL-6 may increase the number of dividing plasma cells in cytogenetic cultures and that a 2-day culture with these cytokines may facilitate the detection of chromosome abnormalities in MM.