Renal tubular function was evaluated in nine patients undergoing recombinant interleukin 2 (rIL2) treatment for metastatic colorectal carcinoma. A lithium clearance technique was used and the activities of the lysosomal enzyme N-acetyl-βD-glucosaminidase were also measured in the patients' urine, before treatment, during treatment, and then 2 days and 23 days after rIL2 therapy had finished. Significant reductions in clearances of creatinine, sodium, and lithium were observed. The fractional excretions of sodium and lithium were also reduced. Twenty-three days following cessation of rIL2 treatment, there was still a significant reduction in creatinine clearance compared with pretreatment values (p < 01). The clearances of sodium and lithium were also reduced compared with pretreatment values although this did not achieve significance. The fractional reabsorption of sodium and water by the proximal nephron increased during rIL2 treatment, from 0.707 ± 0.030 (pretreatment) to 0.793 ± 0.043. This increased reabsorption of sodium and water persisted, rising to 0.849 ± 0.029, 2 days following cessation of treatment (p < 001, means ± SEM). Twenty-three days later this had returned toward the pretreatment value, being 0.781 ± 0.036. The fractional reabsorption of sodium by the distal nephron was also significantly elevated, both during and 2 days after completing rIL2 treatment. Twenty-three days after cessation of rIL2, this value had returned to the pretreatment value. However, in contrast, the fractional reabsorp-tion of water by the distal nephron demonstrated no change during rIL2 treatment, but 2 days posttreatment was significantly reduced and remained low for a further 3 weeks.