The subcellular distribution of the zearalenone-reducing activity in liver from female pig, goat, sheep, cow and hen was investigated. The distribution patterns for the reduction of zearalenone to alpha- or beta-zearalenol differed between species and was also dependent upon coenzyme. Pig and goat had the greatest ability to form both alpha- and beta-zearalenol in the microsomal fraction independently of coenzyme. Cow and hen formed alpha-zearalenol almost entirely in the microsomal fraction and beta-zearalenol only in the cytosol fraction and only with NADPH as coenzyme. The sheep was distinct from the pig and goat in having the highest alpha-zearalenol forming activity in the cytosol fraction when NADPH was used as coenzyme.