Two cases are described of severe Mycoplasma pneumoniae infection in whom cross-reacting antibodies developed to a glycopeptide antigen of Micropolyspora faeni. The antibodies were predominantly IgG, although some IgM was detected. Precipitin responses to M. faeni were detected for 4 months after the initial illness in one case, and 1 month in the second case. The cross-reacting antibodies were closely related to the antibodies responsible for the complement fixation reaction to M. pneumoniae. It is suggested that a common potysaccharide antigen may exist on the surface membranes of M. pneumoniae and M. faeni. Initial studies of the frequency of the development of cross-reacting antibodies in M. pneumoniae infection suggest it is uncommon. Atypical cases of farmer's lung should be investigated to exclude M. pneumoniae infection.