Protein A-rich staphylococci coated with Haemophilus influenzae type b antiserum agglutinate specifically with homologous bacterial cells or with cell-free supernatant fluids of cultures of the organism. Antibody-coated staphylococci were used to detect soluble antigens in body fluids of patients infected with H. influenzae type b. Cerebrospinal fluid from 36 cases of meningitis caused by this orgainsm showed positive coagglutination tests in 86% of patients prior to initiation of therapy. Antigens could be detected in 46% of sterile cerebrospinal fluid specimens obtained from the same cases 1 to 10 days after therapy. Soluble antigens were also detectable in sera (58%) and urine specimens (67%) of patients with H. influenzae type b septicemia, when such specimens were tested within 10 days of onset of illness. No antigen could be detected in body fluids beyond 10 days. The coagglutination test was positive in 57% of all body fluids examined; contercurrent immunoelectrophoresis (CCIE) was positive in only 27%. All specimens positive by CCIE were also positive by coagglutination. No false-positive reactions were noted by either test in body fluids from controls. The coagglutination test is simple, specific, and more sensitive than the CCIE method and could be a valuable tool for detecting antigens in body fluids of patients with various infections.