When human umbilical arteries and veins were challenged with calcium ionophore A23187 they released a leukotriene (LT)-like material. On bioassay using guinea-pig ileal smooth muscle, the release was equivalent to 37.4 +/- 7.5 ng LTD4 (mean +/- SEM) per gram of arterial tissue, and 29.0 +/- 4.7 ng LTD4/g of venous tissue. The LT was further purified by high pressure liquid chromatography, and characterised as LTC4 by bioassay and radioimmunoassay of purified fractions. Longitudinal strips from human umbilical arteries in vitro did not contract to doses of synthetic LTD4 in the range 5 X 10(-11) to 10(-9) mol.