Pili are a virulence factor for gonococci and piliated gonococci are more virulent for man and animal models than gonococci devoid of pili. Pili facilitate gonococcal attachment to human cells, and piliated gonococci resist phagocytosis more than non-piliated organisms. Antibody to pili inhibits gonococcal attachment and increases phagocytosis. These effects are maximal when the antibody is directed against antigenically homologous rather than heterologous gonococcal pili. An excess of pili receptor inhibits attachment of pili to human cells, independent of the antigenic type of gonococcal pili used. The molecular basis of the above roles for pili in the pathogenesis of gonorrhea may be better understood by chemically and structurally characterizing pili and the pilus receptor. This includes determination of the complete amino acid sequence of pili from one or more strains of gonococci. Peptides from proteolytic and chemical cleavage of pili will be purified and sequenced using column chromatography, high voltage electrophoresis, paper chromatography, and sequential Edman degradation. Correlation of this sequence with previous results from modifications of specific amino acids that affected pilus attachment function, may identify site(s) that interact with the pilus receptor. Analysis of the antigenicity of selective proteolytic or chemical cleavage fragments of pili (i.e. with TPCK-trypsin or BNPS-Skatole) may identify the antigenic portion(s) of the pilus molecule. Quantitative immunoassays will be used to determine the extent of shared antigenicity among pili, and to identify antigenic cleavage fragments of pili. A precise chemical characterization of the pilus receptor, and of the speicficity of pili for this receptor, should determine whether gonococcal pili can be utilized as a probe to quantitate the amount of pilus receptor on human cells, and may help to explain the cellular specificity of human gonorrhea. Finally these studies may provide a model for understanding pili mediated attachment of other gram negatve bacteria to human cells.