The proliferation of 5T2 MM cells as quantified by³H-thymidine incorporation is expressed as CPM (counts per minute). 5T2 cells were incubated alone, on bone marrow fibroblasts, in bone marrow fibroblasts–conditioned medium, in the presence of 10 ng/mL interleukin-6 or IGF-1, and on skin fibroblasts (SK). Results represent the mean ± SD of quadruplets. This experiment is representative of three independent experiments.