SummaryLung tissue from human fetuses, with gestational ages ranging between 14–26 wk, was studied by histochemical and biochemical methods. The findings were similar in all tissues tested, without apparent correlation to gestational age. Staining that indicated carbonic anhydrase activity was found in the capillary endothelium and in the epithelium of some segments of the peripheral airways. The ciliated epithelium of the central airways was unstained. The distribution of the enzyme in the human fetal lung differed clearly from that in the adult human lung, where little or no enzyme has been found in the airway epithelium. The mean carbonic anhydrase activity in whole homogenates of fetal lung tissue was 24 enzyme units per g wet weight of tissue. Ninety % of this activity was recovered in the supernatant fraction. Assay of this fraction by a radioimmunosorbent technique showed the presence of the carbonic anhydrase isoenzyme HCA-C corresponding to 380 ng enzyme per mg tissue protein. Small amounts of HCA-B were also found but are thought to be attributable to contaminating erythrocytes; thus, the data suggest that both the capillary endothelium and the lung epithelium contain HCA-C, an isoenzyme of carbonic anhydrase known to be involved in electrolyte transport in many tissues.