The metabolic fate of L-[35S]cysteine was investigated in growing rats fed on diets containing graded levels of protein calorie percentages (0, 5, 10, 15, and 30 PC%) by use of purified whole egg protein at 4,100 kcal of metabolizable energy per kilogram of diet. Incorporation of radioactivity into body protein during the 12 h period after intraperitoneal injection of 35S-cysteine was about 70% of the dose in the 0 to 15 PC% group, but it decreased significantly in the 30 PC% group, showing a break point at around 15 PC% in the diet. A considerable amount of the radioactivity was recovered in the carcass soluble fraction in which the label in the taurine fraction gradually increased with increasing dietary protein levels from 0 to 30 PC%. Urinary excretion of total 35S during the 12 h period was depressed in the lower PC% groups, but it increased in the 30 PC% group, about 23% of the dose being recovered. More than 50% of the urinary radioactivity was present in the inorganic sulfate fraction, but less than 10% was in the taurine fraction. These results indicate that cysteine sulfur is preferentially utilized for body protein synthesis, especially in dietary protein depletion, and that the oxidation of cysteine sulfur to taurine and inorganic sulfate is elevated with higher PC% in the diet. The response pattern of cysteine sulfur metabolism to dietary protein intake resembled that of cysteine carbon metabolism which was previously reported.