Pili were isolated from two different strains of gonococci (33 and 2686) and demonstrated to be pure by the criteria of electron microscopy and polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Each purified pili preparation was studied for its ability to inhibit; (a) binding of 124-I-labeled purified 2686 pili by antibody to 2686 pili, and (b) binding of 125-I-labeled purified 33 pili by antibody to 33 pili. In each instance progressive inhibition of binding was produced by homologous pili type, but no significant inhibition was observed using comparable weights of the heterologous pili type. These results indicate that the pili of strains 2686 and 33 are antigenically different.