Natural suppressor (NS) cell line (Clone 59) was established from the bone marrow of adult C3H/Hej mice in the presence of WEHI-3 conditioned media. Clone 59 cells suppressed the generation of cytotoxic T lymphocytes from normal mouse spleen. This suppression was seen at a responder-to-suppressor cell ratio of 1000:1 and lacked antigen specificity or MHC restriction. Clone 59 cells expressed the 'null' surface phenotype (Thy1.2-, CD3-, Lyt-2-, L3T4-, surface Ig-, MAC-1-) by immunofluorescent staining. Clone 59 cells exhibited no cytolytic activity against NK cell-sensitive YAC-1 and natural cytotoxic L929 target cell lines. Non-specific suppression, with a cell-free supernatant from the Clone 59-NS cells, also was observed. The supernatant did not inhibit [3H]-thymidine uptake by CTLL-2 cells which were proliferating in response to IL-2. Anti-transforming growth factor-beta (TGF-beta) monoclonal antibody had no effect on suppression, suggesting that the non-specific suppression is mediated by some soluble factors other than TGF-beta. Clone 59 cells may be useful in identifying non-specific suppressor cells in adult bone marrow and studying their functional role in the regulation of tolerance and self-reactivity.