IL-3 expression levels by purified CD138⁺ plasma cells from MM patients and healthy donors. CD138⁺ plasma cells were purified from patients with MM and healthy donors. First, 2 × 10⁶ cells were cultured in RPMI media containing 10% fetal calf serum for 96 hours. IL-3 expression levels in these culture media were measured by commercially available IL-3 ELISA kits. At the end of culture period, cells were harvested, and mRNA expression levels of AML-1A and AML-1B were determined by RT-PCR analysis. Three of 6 purified CD138⁺ plasma cells from MM patients who showed enhanced AML-1A mRNA expression secreted 26 to 95 pg/mL IL-3 into culture media. IL-3 was not detected (ND) in media conditioned by CD138⁺ cells from healthy donors and patients 2, 5, and 6 (P2, P5, and P6).