Myelopeptide 1 (MP-1, Phe-Leu-Gly-Phe-Pro-Thr) and myelopeptide 2 (MP-2, Leu-Val-Val-Tyr-Pro-Trp) are hexapeptides originally isolated from porcine bone-marrow cell culture. MP-1 enhanced the number of antibody-forming cells in NZB mouse spleen cells in vitro and increased the antibody production in Cy-treated (CBAxC57B1/6)F1 mice in vivo. MP-1 increased ConA-induced proliferation of both NZB spleen cells in vitro and spleen cells of Cy-treated mice in vivo. MP-1 restored polyclonal IgG synthesis depressed by PWM in the NZB spleen cell culture. MP-2 also evoked immunoregulatory effects which were less pronounced and realized at higher concentrations than effects of MP-1. Neither MP-1 nor MP-2 affected LPS-induced spleen cell proliferation in both NZB- and Cy-treated (CBAxC57B1/6)F1 mice.