Mycoplasma pneumoniae, a respiratory pathogen of humans, is the etiologic agent of primary atypical pneumonia. The mycoplasma attaches to the host cell by means of a specialized terminal structure. This structure binds the organism to the corresponding receptor site on the host cell. A glycoprotein receptor site for M. pneumoniae was isolated from MRC-5 human lung fibroblasts. It was isolated from a Triton X-100 extract by column affinity chromatography, utilizing wheat germ agglutinin bound sepharose 6MB. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed it to have an MW of approximately 100 kilodaltons (kDa) and to be composed of subunits of approximately 21 kDa. At a concentration of 56 micrograms/ml, the receptor inhibited the binding of 14C-labeled M. pneumoniae to MRC-5 fibroblasts by 77%. Chemical analysis determined that it does not contain any detectable sialic acid. Knowledge of the biochemical and immunological characteristics of the receptor site are essential to understand the attachment phase of the pathogenic process and to further the development of effective prophylaxis.