The MOPC 104E cell line has been adapted to grow in vitro using a combination of feeder layer and growth factors). The growth of this myeloma cell line is dependent on the presence of growth factors). Growth-promoting activity generated from T-cell-mitogen-stimulated, Corynebacterium parvum-stimulated spleen cell culture supernatant, and peritoneal adherent cell culture supernatants gives dose-dependent proliferation. Generation of growth factors in the serum-free bovine serum albumin-substituted media and a rapid assay system based on [3H]thymidine uptake for the quantitation of growth promoting activity are described. © 1985.