The two epimers of [3H]dihydromicrocystin-LR (spec. act. 32.6 microCi/mumole) were synthesized by reduction of microcystin-LR, a cyclic heptapeptide hepatotoxin, with sodium boro[3H]hydride. In Atlantic salmon, following an i.p. injection of the epimers (0.1 mg of toxin/100 g), the toxins were; after 2 hr, concentrated in the liver and pyloric caeca and intestine; after 5 and 22 hr, concentrated in the liver; and after 46 hr concentrated in the liver and kidney. The maximum concentration in the liver, 4.9 +/- 0.5% of the injected dose, was observed at 5 hr. At 46 hr the incorporation of tritium in the liver was 2.4%. The total whole body incorporation of tritium label at 2, 5, 22 and 46 hr was 67.9 +/- 7.3%, 78.4 +/- 3.1%, 54.5 +/- 6.6% and 12.2%, respectively. The per cent levels of incorporation are significantly lower and the clearance rates are significantly higher than those observed in mice. The ratio of relative concentrations of radiolabel observed in the livers and flesh indicates that extremely low levels of microcystin-LR would be present in the flesh of fish afflicted with 'netpen liver disease'. Coupled with further evidence, it is suggested that the consumption of Atlantic salmon suffering from 'netpen liver disease' does not represent a potential human health hazard.