A new procedure is described for the purification of gonadotropic hormones (GTHs) from the pituitary glands of vitellogenic rainbow trout. The procedure utilizes immobilized metal ion affinity chromatography (IMAC) on a column containing immobilized iminodiacetic acid (Toyopearl AF Chelate) charged with Cu2+ ions as a critical step for the efficient separation of GTH I and GTH II. Further purification of both GTH fractions on Cibacron Blue F3GA immobilized on Toyopearl was followed by HPLC size-exclusion for GTH II. The resulting electrophoretically homogeneous preparations possessed a characteristic range of biological activity in the stimulation of steroidogenesis in vitro. N-Terminal sequences of the GTH I and GTH II subunits purified using reversed-phase HPLC revealed a high level of homology with those of GTH subunits found in three other salmonid fish species. In contrast to salmon GTH II, two different alpha-subunits were observed in trout GTH II. The cross-reactivity between GTH I and GTH II was studied in radioimmunoassay using antibodies against Chinook salmon GTH II beta-subunit and rainbow trout GTH I dimer. (C) 1997 Elsevier Science B.V.