Raised cold agglutinin titres were observed in 16 patients with atypical pneumonia. In 7 cases serological demonstration of antibodies against Mycoplasma pneumoniae (complement binding reaction) was possible. In 4 cases the causative organism could not be demonstrated serologically and in 5 cases the demonstration of antibodies was not possible. There were 11 sporadic single cases and 5 patients from a mycoplasma family infection. The incubation time was between 7 and 16 days. History, clinical symptoms and the auscultatory findings in connection with the radiograph allow a preliminary diagnosis of primary atypical pneumonia which can be finally proved by the demonstration of cold agglutinins. The titres reach their highest levels within 2-3 weeks and fall again within 4-6 weeks. As the complement binding reaction with Mycoplasma pneumoniae is still positive after several weeks the diagnosis can also be proved retrospectively. Reconvalescence is markedly delayed and lasts several weeks. Clinically and therapeutically the cold agglutinin positive atypical pneumonias cannot be differentiated from mycoplasma pneumonias.