Normal human plasma and plasma from patients with chronic lymphocytic leukemia (CLL) were used as growth supplements for the cloning of murine hybridomas. Basal medium, consisting of DMEM with 5% foetal calf serum (FCS) was conditioned with supernatant from a known human IL-6-secreting cell line, BRI-6 (BRI-6-CM), normal human plasma (NHP-DMEM), and plasma from patients with CLL (CLL-DMEM). When compared to conventional feeder layers of macrophages, thymocytes, splenocytes and to feeder CLL cells and BRI-6-CM the numbers of clones formed by growing hybridomas in CLL-DMEM was greatly enhanced with a corresponding increase in the number of antibody-producing clones, as determined by ELISA. NHP-DMEM also enhanced the cloning efficiency. All CM and plasma supplemented medium were examined for the presence of interleukin-6 (IL-6). Of eight CLL plasma samples examined only two had elevated IL-6 levels.