The function of the Rhesus (Rh) complex in the human red cell membrane has been unknown for six decades. Based on the organismal, organ, and tissue distribution of Rh proteins, and on our evidence that their only known paralogues, the ammonium and methylammonium transport proteins (also called methylammonium permeases), are gas channels for NH(3), we recently speculated that Rh proteins are biological gas channels for CO(2). Like NH(3), CO(2) differs from other gases in being readily hydrated. We have now tested our speculation by studying expression of the RH1 gene in the photosynthetic microbe Chlamydomonas reinhardtii. Expression of RH1 was high for cells grown in air supplemented with 3% CO(2) or shifted from air to high CO(2) (3%) for 3 h. Conversely, RH1 expression was low for cells grown in air (0.035% CO(2)) or shifted from high CO(2) to air for 3 h. These results make viable the hypothesis that Rh1 and Rh proteins generally are gas channels for CO(2).