We established a radioimmunoassay of 6-keto PG F1 alpha, using 125I-6-keto PG F1 alpha-tyramide, which enabled us more easily to monitor the plasma levels of prostacyclin in clinical fields. Antibodies against 6-keto PG F1 alpha were obtained from rabbits immunized with 6-keto PG F1 alpha-BSA complex, which could be used for RIA at a final dilution of 1:648,000. The detection limit of 6-keto PG F1 alpha was 50-1000 pg/ml. The cross reactivities between PG I2 and PG F1 alpha were 26.6% and 1.62% respectively, but those between other prostaglandins were less than 1%. Using this radioimmunoassay with 125I-6-keto PG F1 alpha-tyramide, plasma levels of 6-keto PG F1 alpha were measured. The recovery rate by this assay was 92.9%, and the intra- and inter-assay coefficients of variation determined were 9.9% and 27.1%, respectively. The mean of plasma levels of 6-keto PG F1 alpha revealed 126.2 +/- 75.3 (mean +/- S.D.) pg/ml in 46 healthy Japanese males, 122.7 +/- 85.2 pg/ml in 60 females, and 124.2 +/- 81.3 pg/ml in 106 Japanese of both sexes.