Growth studies were conducted with fingerling channel catfish to determine the minimal requirements for the amino acids leucine, isoleucine, valine and histidine. The experimental diets were formulated from purified ingredients to contain 24% crude protein and 275 kcal/100 g diet. Casein and gelatin served as protein sources and were supplemented with crystalline L-amino acids to provide the amino acid pattern found in 24% crude protein from whole egg powder. Based on dry diet the catfish required 0.84 +/- 0.04% leucine, 0.62 +/- 0.04% isoleucine, 0.71 +/- 0.03% valine and 0.37 +/- 0.01% histidine. These values correspond to 3.50% leucine, 2.58% isoleucine 2.96% valine and 1.54% histidine when expressed as a percentage of dietary protein. Quantitation of serum free amino acid levels revealed a marked increase in valine and isoleucine in leucine-deficient fish. In addition, increases in dietary isoleucine or valine resulted in changes in the serum levels of isoleucine, valine and leucine. These data indicate an apparent interaction among these amino acids in the catfish comparable to that previously observed in other species. Dietary histidine caused no increase in serum free histidine levels until the dietary requirement was reached. Muscle carnosine could not be detected in the catfish.