A search for cellular components responsible for autoagglutination (AA) in broth and salt solutions of Hms- cells of the plague agent Yersinia pestis was performed. The AA- mutants were obtained using vaccine strain Y. pestis EV76 derivative containing one species-specific plasmid pYP. The mutants were shown to differ from the parent strain by the decreased surface hydrophobicity, insensitivity to plague diagnostic L-413c bacteriophage and negative haemagglutination reaction with antibodies to F1 capsular substance of the plague agent. The mutants did not differ from the parent strain by electrophoretic mobility and immunochemical activity of LPS but were characterized by the absence of a 17 kDa protein on the cell surface. The AA+ cells that lost this protein after weak alkali extraction were less hydrophobic and failed to express AA in 0.5 M ammonium sulfate. After the extraction, the cells lost the ability to neutralize L-413c and to react with the anti-F1 antibodies, while both activities as well as 17 kDa protein were detected in the extracts. Thus, the 17 kDa protein is suggested to be a hydrophobic surface antigen which acts as a receptor of the L-413c bacteriophage and represents an AA factor of Hms- cells of Y. pestis.