The presence of carbonic anhydrase isoenzymes I and II in rat monocytes and macrophagelike cells was studied using monospecific antisera against rat carbonic anhydrase I and II purified from red blood cells. CA II was strongly stained immunohistochemically in osteoclasts and macrophagelike cells in the umbilical cord. Foreign body giant cells, peritoneal macrophages, lung macrophages, and cultured peripheral monocytes, the presumed progenitor cells for osteoclasts, were negative with both antisera. Radioimmunoassay and immunoblotting similarly failed to demonstrate CA II in peripheral monocytes. The lack of CA in monocytes adds a new aspect to the discussion concerning the origin of osteoclasts and monocyte-mediated bone resorption.