This study investigated the role of gonadotrophin and steroids in the induction of in vitro oocyte maturation in dab (Limanda limanda) and plaice (Pleuronectes platessa), two marine flatfish (Pleuronectiformes) in which the oocytes mature and ovulate in groups (or batches). Clusters of folliculated oocytes were incubated for 3 days with a range of doses of either human chorionic gonadotrophin (HCG) or synthetic steroids with a wide range of structures. They were also incubated with extracts from cultures of HCG-stimulated ovaries. Oocytes were always more responsive to HCG than to steroids, and the median efficient doses obtained with HCG were negatively correlated with oocyte diameters. Bioassay sensitivity to steroids was low when compared with other teleosts, and intraassay variability was high, particularly in plaice. Despite the high variability, a common pattern in specificities to steroids was obtained for the two species, which was also similar to other teleost species. Among the most effective steroids found were 17 alpha,20 beta-dihydroxy-4-pregnen-3-one and 17 alpha,20 beta,21-trihydroxy-4-pregnen-3-one. Extracts from incubates were ineffective. The significance of these findings and possible ways to reduce assay variability and improve sensitivity are discussed.