Abstract: Water soluble biologically active fraction obtainable from the physiologically inter-cellular medium bone-marrow and process for preparing said fraction and the pharmaceutical compositions containing said fraction. The characteristics are notably the following: it is substantially free of the inhibitors having molecular weights lower than 30,000; it is lyophilizable without loss of biological properties; it exhibits the biological activities in the form of a solution having a pH of at least 7; it stimulates H-thymidin incorporation by bone-marrow cells in vitro and in vivo; the water soluble fraction is capable of controlling the immune reactions of a host against allogenic cells or tissue.