The relative growth-stimulating properties of various animal fractions were measured in vitro using a rainbow trout gonadal cell line. No significant differences were noted at the end of a 12 day growth period when 10% of either human serum, bovine serum, or ascitic fluid was added to the basal medium. Ascitic fluid stimulated early growth of the cells but did not result in greater final numbers and apparently caused early degeneration of the cell. Fetal calf serum, bovine embryo extract, and human or bovine serum were required for maximum proliferation of rainbow trout gonadal cells in these experiments.