Male Beagles infected with Brucella canis for greater than or equal to 3 months developed serum antibodies that agglutinated normal canine spermatozoa. Titers were highest in dogs that had been infected for 4 to 6 months. Lower spermagglutinin titers were detected in sera collected 10 months after inoculation. Antibodies were also observed in seminal plasma of chronically infected dogs. Seminal plasma from infected, but not from clinically normal dogs, caused head-to-head agglutination of normal sperm. In contrast to macroagglutination of sperm by serum antibodies, agglutination by seminal plasma antibodies was detected only by microscopic examination. Seminal plasma agglutinins were not inactivated by heat (56 C, 1 hour) or by reduction with 2-mercaptoethanol. When seminal plasma and sperm were mixed with 2 hemolytic units of guinea pig complement, spermatozoa were not inactivated. Spermagglutinin activity was present in the first 2 spectral absorption peaks of Sephadex G-200 fractionated seminal plasma. Fractions that had the highest spermagglutinin titers contained mostly immunoglobulin A. Seminal plasma from infected dogs also contained cytophilic factors for normal splenic macrophages that caused sperm adherence to macrophages. Dogs with a bacteremia lasting greater than 4 months had cutaneous delayed-type hypersensitivity reactions when tested with soluble canine testicular extracts. Reactions did not occur in normal dogs. Dogs with testicular atrophy had the most severe skin test responses. Seemingly, isoimmune responses to sperm antigens are involved in infertility caused by B canis infection of male dogs.