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Recombinant soluble CD4 (sT4; mol. wt. 45,000) has been studied extensively in Sprague-Dawley rats, and substantial renal processing has been indicated. In rats and monkeys, renal filtration and precipitation of sT4 in the distal nephron caused tubular cast nephropathy. Intravenous pharmacokinetics in the rat demonstrated that sT4 plasma clearance exceeded the glomerular filtration rate. In an effort to determine quantitatively the extent to which kidney and other tissues were responsible for sT4 catabolism, sT4 was labeled with trace amounts of dilactitol-[125I]tyramine and administered intravenously to Sprague-Dawley rats (1 mg/kg). Dilactitol-tyramine accumulates in lysosomes at the site of protein degradation. It has been used primarily to demonstrate hepatic catabolism of endogenous proteins. Blood samples were drawn for pharmacokinetic analysis, and selected tissues were removed to assess radiolabel distribution. Comparison of pharmacokinetic parameters derived from total plasma radiolabel and functional ELISA were not significantly different. Thus, covalent modification of sT4 with dilactitol-tyramine did not appreciably change the rate of clearance. From 3 to 24 hr after intravenous administration, 81.5 +/- 0.1% of the total administered radioactivity was found in the kidney. Approximately 8-13% of the administered dose was recovered in the liver. Macroscopic autoradiography of the kidney demonstrated accumulation of radiolabel in the cortex. Light microscopic autoradiography of the kidney following intravenous administration of directly radioiodinated sT4 confirmed cortical processing, because radiolabel was located primarily in epithelial cells of P1 and P2 segments of the proximal tubule after low intravenous doses (0.4-4 mg/kg). At 40 mg/kg, distal tubules and cortical collecting ducts were labeled as well. Thus, sT4 was filtered by the glomerulus, reabsorbed in the proximal tubule, and degraded in the lysosomal compartment.