A microtechnique complement-fixation (CF) test was used to establish identification groups among the previously described avian mycoplasma serotypes. The three named pathogenic species, Mycoplasma gallisepticum (serotype A), Mycoplasma meleagridis (serotype H), and Mycoplasma synoviae (serotype S), were each separable into a well-defined antigenic group, as were serotypes F and L. Predominantly one-way cross-relatedness was demonstrable between the other two named species included in this study, Mycoplasma gallinarum (serotype B) and Mycoplasma iners (serotype G). Serotype E was indistinguishable from serotype G by CF test. Crossrelatedness was also demonstrated among serotypes C, D, 0, and P and among serotypes I, J, K, N, Q, and R.