The effect of protein intake (10 vs. 20%) on synthesis and degradation of short-lived leukocyte proteins in vitro was shown in 21-day-old chicks. Protein synthesis was measured for 1 or 2 h and protein degradation for 1.5 or 4 h following an 8-day dietary treatment. Leukocyte proteins were fractionated on a slab gel into 11 or 100 fractions, and the imaging radioactivity was calculated for protein synthesis and degradation. The degradation rates in most of the protein fractions were higher in the leukocytes of chicks fed 20% dietary protein than in those fed 10% protein. Protein fractions of M(r) approximately 25-37 kD exhibited the most marked differences between the two dietary treatments. This technique of measuring turnover rates of short-lived proteins in leukocytes could potentially be used to examine the effect of nutritional treatments in animals and man as well as to check the effect of catabolic conditions in humans on protein turnover.