Progesterone-treated lymphocytes (generator lymphocytes) of healthy pregnant women release a nondialyzable factor that inhibits both cytotoxic activity and prostaglandin F2 alpha synthesis of test lymphocytes. Production of this factor is blocked by protein synthesis inhibitors (cycloheximide and actinomycin D). Sodium dodecylsulfate polyacrylamide electrophoresis of the partially purified material revealed a main 34,000 MW protein band. Progesterone-treated lymphocytes of pregnant women showing clinical symptoms of threatened preterm delivery (risk group) failed to release this substance.