Human carbonic anhydrase [HCA] activity was studied in the ciliary process of fresh human donor eyes using a new radioimmunosorbent technique for the quantitive assay of B-and C-types of HCA. HCA originates from an enzyme antigenically similar to the erythrocyte high-activity isoenzyme HCA C. It was sensitive to inhibition by acetazolamide and resistant to inhibition by halides like HCA C. The enzyme is probably identical with HCA C. Its tissue concentration was 1/5-1/10 of that in the human kidney. The erythrocyte low-activity isoenzyme HCA B was also found in the processes as a contaminant.