A new method was developed for the isolation of biologically active peptides present in small quantities in supernatants of bone marrow cell cultures. Four new peptides, which exhibited immunostimulating and differentiating activities, were isolated from the supernatant using solid phase extraction and reversed-phase HPLC. For these peptides, the following primary structures were elucidated: Leu-Val-Cys-Tyr-Pro-Gln, Phe-Arg-Pro-Arg-Ile-Met-Thr-Pro, Val-Val-Tyr-Pro-Asp, and Val-Asp-Pro-Pro.