Human myometrium obtained at caesarean sections from term pregnancies was homogenized and separated into nuclear, mitochondrial, microsomal and cytosol fractions. Purity of the individual fractions was assessed by microscopy and by determination of marker enzymes. Homogenate and subcellular fractions were incubated with [3H]-oestrone sulphate in a concentration of 5 x 10(-10) M after addition of NADPH2 and a coenzyme regenerating system at 37 degrees C. Incubation were terminated after 10, 20, 40, 60, 120 and 180 min. Substrate and metabolites were characterized by repeated chromatographies on Sephadex LH 20 using various solvent systems. Further characterization of the metabolites was achieved by simultaneous chromatography of the isolated compounds and their authentic cold standards. The enzymes involved in the metabolism of oestrone sulphate to oestrone and oestradiol-17 beta are primarily located in the microsomal fraction. After 40 min 71.1%-73.0% was converted to oestrone, while the conversion rate to oestradiol-17 beta was in the order of 1.38-1.90%. Metabolism of oestrone sulphate seen in the other subcellular fractions is probably due to microsomal contamination. The findings presented indicate that human myometrium is capable of converting oestrone sulphate to free oestrone and oestradiol-17 beta under in vitro conditions.