A myelopeptide (SAP) was derived from culture supernatants of unstimulated animal bone marrow. SAP consists of a group of peptides with a molecular weight of about 2000 D, having a broad variety of biological activities. Testing immunoregulatory properties of the purified factor Petrov, Mickhailova & Zacharova [(1971). Immunoglobin synthesis in syngenic cells of different lymphoid tissues. J. Immun., 106 1086-1089] found enhanced antibody production in mice (SAP-stimulator of antibody production). We show here that the substance could induce expression of activation markers on human lymphocytes (4F2, HLA-class II antigens, thermostable SE rosette formation) and potentiate their appearance in combination with mitogens (PWM, PHA, Con A). Although SAP was not mitogenic for itself, it enhanced lectin-induced 3H-thymidine incorporation and T-cell-dependent B-cell differentiation in a dose-dependent manner. The factor was able to reconstitute disturbed PWM-driven Ig synthesis in lymphocyte cultures derived from two patients with hypogammaglobulinemia and a healthy non-responder to PWM. On the other side, SAP potentiated the inhibitory activity of PWM on elevated spontaneous IgG secretion in cultures derived from patients with active SLE. Findings of this study indicate immunomodulatory capacity of SAP on human peripheral blood lymphocytes possible via T-cell activation. The results suggest a potential therapeutic application of SAP in patients with disturbances in the T-dependent B-cell differentiation.