Extraction of the S-layer protein by treatment with 6 M urea revealed a high-molecular-weight protein in the extracts obtained from Bacillus thuringiensis subsp, israelensis (B.t.i.) strain 4Q2. This protein band was found to be absent in partially cured (4Q2-72) and completely cured (c4Q2-72) strains. The antibody toward this S-layer protein was prepared and used to locate its antigenic protein on B.t.i cells by using indirect immunofluorescence. Immunodiffusion reactions and Western blot analysis confirmed the specificity of the anti-S-layer protein antibody. It was found that the antibody against 4Q2 S-layer protein inhibited plasmid transfer via a conjugation-like process between B.t.i. strains 4Q2-16 and c4Q2-72. That is, the frequency of transfer of plasmid pBC16 was reduced from 9.7 x 10(-6) in the absence of the antibody to less than 1.0 x 10(-8) in the presence of the antibody. The antibody was also found to reduce the frequency of pBC16 plasmid transfer via a conjugation-like process between B.t.i. strains A084-16-194 and c4Q2-72 from 2.2 x 10(-5) in the absence of the antibody to 1.2 x 10(-6) in the presence of the antibody.