Abstract: Solubilized antigens of C. trachomatis strain LGV-434 upon analysis using two-dimensional immunoelectrophoresis yielded a single antigen which was found to be consistently precipitated by sera of patients with C. trachomatis infections. This antigen as antigen-antibody complex was employed as an immunogen to prepare a rabbit monospecific antiserum to this component or antigen. This monospecific antiserum demonstrated the presence of the antigen in each of the 15 strains of C. trachomatis organisms and was non-reactive with strains of C. psittaci. The C. trachomatis specific antigen was purified by immunoadsorption chromatography using monospecific antiserum as a specific ligand covalently bound to agarose gel columns and the resulting purified antigen employed to detect antibody from the sera of lymphogranuloma venereum patients using counterimmunoelectrophoresis. When the C.