Tyrosine was incorporated into highly purified porcine relaxin employing the reagent N-carboxy-L-tyrosine anhydride. The resulting polytyrosyl-relaxin contained 1.67 mol of tyrosine per mol of relaxin, retained its original biological activity, and was readily radioiodinated to specific activities ranging from 80 to 100 muCi per mug. High affinity antibodies applicable in final dilutions ranging from 1:50,000 to 1:200,000 were developed in rabbits against unconjugated highly purified porcine relaxin. A double antibody radioimmunoassay for porcine relaxin sufficiently sensitive to routinely measure from 32 to 1000 pg of pig relaxin was developed. Using this radioimmunoassay, peripheral serum concentrations of porcine relaxin were found to be less than 1 ng/ml during early pregnancy. Serum concentrations of porcine relaxin were high during late pregnancy. The mean concentration of porcine relaxin one day before parturition was 38 ng per ml. Within a day following parturition relaxin concentrations fell to a mean concentration of 2.1 ng per ml.