Cold agglutinin (anti-I) titres in patients with the chronic cold agglutinin syndrome and in patients with M. pneumoniae infection have been shown to bear no relationship with growth-inhibiting and complement-fixing antibody titres against this organism. These findings, together with the inability to absorb anti-I agglutinin at 4° with M. pneumoniae antigen, suggest that M. pneumoniae does not contain red cell I antigen.