SummaryA chloroform‐methanol extract ofMycoplasma pneumoniaereacted with bovine antisera toMycoplasma mycoidesin a complement fixation test. Some antisera gave higher titres with theM. pneumoniaeantigen than with the homologous antigen; absorption of the antisera withM. mycoidesantigen reduced the titres to either antigen by approximately the same amount. Absorption of antisera withM. pneumoniaeantigen greatly reduced the titre of antisera toM. pneumoniaeantigen but the titre to the homologous antigen was reduced fourfold or less. It is concluded that only some of the complement fixing antigens ofM. mycoidesare shared withM. pneumoniae.