Although the kidney is the critical organ limiting occupational exposure to soluble uranium compounds, there have been no adequate studies evaluating renal tubular dysfunction in chronically exposed workers. The present investigation evaluated kidney function among 39 uranium mill workers and 36 local cement plant workers of equivalent age, sex, and race. The uranium workers showed a significantly higher excretion of beta-2-microglobulin and five amino acids than the reference group. Although the levels of tubular proteinuria were mild, a dose-effect relation existed between the clearance of beta-2-microglobulin, relative to that of creatinine, and the length of time that the uranium workers had spent in the yellowcake drying and packaging area, the work area with the highest exposures to soluble uranium. Age did not account for this relationship. Glomerular function was significantly better among the uranium workers than among the referents, though this may have been the result of differences in the physical activity of the groups during the collection period. The data presented suggest reduced renal proximal tubular reabsorbtion of amino acids and of low molecular weight proteins, consistent with uranium nephrotoxicity.