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Previous epidemiologic studies have shown that most primary atypical pneumonia illnesses in which cold agglutinins develop are associated with the agent first described by Eaton, Meiklejohn, and van Herick in 1944 (1-8). In addition the agent causes a spectrum of effects ranging from inapparent infection to febrile respiratory disease without pneumonia (5, 6).Recent studies have established that the organism, previously known as "primary atypical pneumonia virus" or "Eaton agent", is not a virus but a member of the genus Mycoplasma (pleuropneumonia-like organisms) (9-11). Thus, at least 30 strains have been grown in cell-free semisolid or liquid medium containing bovine heart infusion, yeast extract, and horse serum (10-13). Growth does not occur in the absence of serum or a suitable substitute such as egg yolk (10, 14). The colonies which grow on semisolid *agar medium exhibit a colonial morphology and fine structure characteristic of Mycoplasma (10). Certain microbial inhibitors such as thallium acetate, penicillin, and amphotericin B do not affect growth of the organism (10, 11). The agent is inhibited, however, by the tetracycline group of antibiotics (15).Until recently only four species of mycoplasma were known to infect man. These are M. hominis type 1, M. hominis type 2, M. salivarium, and M. fermentans. (16-18). When the atypical pneumonia organism was compared with these species by immunofluorescence or complement-fixation tests it was antigenically distinct (10, 19-21). It resembles M. fermentans in utilizing glucose and other sugars (22). The agent differs biologically from the four recognized human species of Mycoplasma by its ability to produce rapid and complete hemolysis of guinea-pig and horse red cells (23, 24). Under 662 R. M. CHANOCK

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