In renal exploratory excisions the isoenzyme C of the carbonic anhydrase was detected in 51 operative treated patients with recurrent urolithiasis and in 6 patients with ren mobilis or stenosis of the ureteral pelvic junction by using the immunohistochemical PAP-method. In the distal tubules and collecting ducts there is an alternating occurrence of cells with small and those with strong reactivity. This correlates with the distribution of mean or principal (P-, small reaction) and intercalated (I-, strong reaction) cells. The number of carbonic anhydrase rich cells and the degree of their cytoplasmic reaction to this enzyme seem to correlate with the ability of the urine to acidify. The patients were divided into four groups according to the mean urine pH estimated for a period of two weeks. The third group with predominantly acid urine (pH less than 5.8) showed a significantly increased number of carbonic anhydrase rich cells (mean = 63% +/- 6, n = 18) as compared to the first group (controls) without urolithiasis and normal urine (pH 5.8-6.8, mean = 46% +/- 8, n = 6) or the second group with urolithiasis and normal urine (pH 5.8-6.8, mean = 46% +/- 8, n = 28). The fourth group with predominant alkaline urine (pH greater than 6.8) showed a significantly decreased number of intercalated cells (mean = 42% +/- 16, n = 5) in comparison to the third group. Indeed, the difference with the control group is not significant but the cytoplasmic reaction of I-cells decreases distinctly in comparison to all other groups.