Amino acid utilization was evaluated in seven children with acute lymphocytic leukemia treated with succinylated Acinetobacter glutaminase-asparaginase. All patients received food p.o. ad libitum and glucose-electrolyte solutions i.v.; four patients received an i.v. amino acid supplement (1.5 g/kg/day). Although all patients were in negative energy balance, there was a significant linear regression between nitrogen balance and nitrogen intake during Days 1 to 7 and Days 8 to 14 of the study. The slope of the regression line, reflecting exogenous nitrogen utilization, was not significantly different from that found in healthy young men ingesting adequate or subadequate energy intakes. The Y-intercept (-210 mg/kg/day) indicated an obligatory nitrogen loss that was much greater than normal. Most of the nitrogen loss was due to urinary excretion. Ammonia and urea accounted for 77 to 91% of the urine nitrogen. Urinary glutamate accounted for 4 to 10% of this loss. Urine protein excretion was abnormally high in each of the patients, ranging from 987 to 3440 mg/day. Urine excretion of N-acetyl-beta-glucosaminidase and beta 2-microglobulin was also abnormally high, despite normal blood urea nitrogen and serum creatinine, suggesting that these children had renal tubular dysfunction. The antileukemic effect of succinylated Acinetobacter glutaminase-asparaginase did not appear to be altered by amino acid supplementation. These data indicate that amino acid supplementation can improve nutritional status in patients treated with succinylated Acinetobacter glutaminase-asparaginase.