Active immunization of four adult pigs with highly purified porcine luteinizing hormone (pLH)--using method of multiside intradermal injections--has been performed and resulted in the production of specific antibodies. Immunization caused prolongation of estrous cycle to 47-49 days in two gilts and to 26 days in the other ones. Obtained anti-pLH pig serum was administered intravenously to 40 day pregnant gilt during 5 days (10 ml of serum, twice daily). Blood plasma progesterone (P4) concentrations decreased significantly from 8-13 to 2-4 ng/ml after two days of infusion and remained at this level for the next 5 days. Administration of this anti-pLH pig serum to gilt in the luteal phase of the estrous cycle caused the inhibition P4 to undetectable amounts. The different results were found after the passive immunization of 40 day pregnant gilt with rabbit anti-pLH globulin preparation (5 days, equivalent to 3 ml of original undiluted serum, twice daily). Although after two days of infusion P4 concentration decreased, in the next days P4 level slowly increased to pretreatment concentrations. The data suggest the possibility of specific anti-pLH antibody production in pigs by using active immunization, and the repeated utilization of such obtained antiserum in the same species for the inhibition of corpus luteum (CL) function.