IL-3 induces osteoclast formation in human marrow cultures and acts at the early stages of OCL differentiation. (A) Long-term human marrow cultures were treated with varying concentrations of recombinant human IL-3 (hIL-3). Controls for these experiments were cultures treated with 1,25–(OH)₂D₃ (10^-8 M). IL-3 at concentrations of 10 to 100 pg/mL significantly increased osteoclast formation in a dose-dependent manner. (B) Human bone marrow cultures were treated with hIL-3 (100 pg/mL) for varying periods of time. IL-3 significantly increased OCL formation only when present during the first 2 weeks of the 3-week culture period. Results represent the mean ± SEM for quadruplicate determinations for a typical experiment. Similar results were seen in 4 independent experiments (^*P < .05), compared with culture lacking IL-3.