The myeloma cell growth factor activity of HB-EGF is inhibited by 2 ErbB1-specific inhibitors.ErbB1–positive myeloma cell lines (XG-5, XG-7, and XG-11) were IL-6 starved for 3 hours and cultured in RPMI 1640 culture medium and 5% FCS with 5 pg/mL IL-6 and 1 μg/mL HB-EGF without (□) or with PD169540 (1 μM; ▪), AG1478 (1 μM; ▨), and PD153035 (1 μM; ▦). Cells were cultured for 6 days and pulsed with tritiated thymidine at the end of the culture. Mean tritiated thymidine incorporations were determined in sixplicate culture wells and results are expressed as percentages ± SDs of the mean proliferation without ErbB inhibitor. *Indicates that the mean value is statistically significantly different from that obtained with 5 pg/mL IL-6 and HB-EGF without inhibitor, using a Student t test (P ≤ .05).