Soluble Mycoplasma pneumoniae antigens were analysed by crossed immunoelectrophoresis. At least 15 precipitation arcs were developed between the soluble antigens and rabbit anti-Mycoplasma pneumoniae hyperimmune sera. Sera from Mycoplasma pneumoniae infected patients formed precipitation arcs which were identified with those formed by a chloroform-methanol extract of Mycoplasma pneumoniae. Low concentrations of human antibodies, not detectable in a complement fixation test, were found to react with these antigens. The antigens were shown to cross-react with antigens of Mycoplasma neurolyticum. Emphasis was placed on the technical development of the crossed immunoelectrophoresis for the analysis of immune-precipitable antigens of Mycoplasma pneumoniae.