A selective clean-up method using an immunoaffinity column followed by radioimmunoassay (RIA) was developed for determining prostaglandin F 2α (PGF 2α) in human urine and plasma. Polyclonal antibody raised against PGF 2α, obtained from rabbits, was coupled to a tresyl-activated support based on a synthetic hydrophilic resin, TSKgel Tresyl-Toyopearl 650M, and used as the stationary phase for the immunoaffinity column. A human urine or plasma sample was introduced to this column, and PGF 2α was eluted with methanol—water (50:50, v/v) after the column had been washed. The eluate was subjected to competitive RIA for PGF 2α. The cross-reactivities of the RIA to a number of endogenous prostanoids, except PGD 2α, were negligible and the sensitivity was 4 pg/tube ( p < 0.05), giving a detection limit of 40 pg/ml when 1 ml of plasma or urine was available. The recoveries of plasma and urine samples were 98–108% and 96–106%, respectively, and their assay variances were 7–23%. The concentrations of endogenous PGF 2α in plasma and urine used here were estimated to be 72 and 98 pg/ml, respectively. This method should be very useful for various biological samples because of its good specificity, sensitivity, reliability and reproducibility.