Tpo is not detected in murine BM cultures induced with rhIL-11 and IL-3. PCR-generated DNA encoding full-length murine c-mpl was subcloned and transfected into FDCP-1 cells by electroporation as described in the Materials and Methods. FDC-MPL cells detected murine Tpo to concentrations of 15 pg/mL. CMs from murine liquid BM cultures were generated by separating out the low-density cells by centrifugation and culturing them with various growth factor combinations for 3 days (20 U/mL rmIL-3 and 10 ng/mL rhIL-11). CMs for assay were collected and cells were stained for AchE activity. FDCP-MPL cells were assayed in microtiter plates at 5 × 10³ cells per well with (A) titration of growth factors ([▪] rmIL-3; [○] rmTpo; [□] media control) or (B) CM with or without antibody to mIL-3 at a 1:100 dilution ([▪] IL-3 CM; [•] IL-3 CM + anti–IL-3; [×] rhIL-11 CM; [□] IL-3 + rhIL-11 CM; [▵] IL-3 + rhIL-11 CM + anti–IL-3) for 72 hours. An enlarged view of the activity detected with (•) IL-3 CM + anti–IL-3, (×) rhIL-11 CM, and the (▵) IL-3 + rhIL-11 CM + anti–IL-3 is shown in the insert graph of (B).