We report the development and testing of an enzyme-linked immunosorbent assay with excellent sensitivity for the detection of Haemophilus influenzae type b (HI(b)) antigen in clinical specimens from patients with HI(b) meningitis. The assay, an indirect sandwich technique, uses polystyrene balls as a solid phase and an alkaline phosphatase-labeled goat anti-rabbit globulin conjugate. Specimens are incubated with polystyrene balls armed with burro anti-HI(b) antiserum, and recognition antibody is visualized by addition of alkaline phosphatase-labeled anti-globulin, together with the enzyme substrate p-nitrophenyl phosphate. Concentrations of antigen are determined from standard curves prepared by using purified HI(b) capsular antigen polyribophosphate. The assay reproducibly detects polyribophosphate at concentrations between 1 and 5 ng/ml. Cross-reactions have not as yet been encountered in simulated and authentic clinical specimens containing other species including Escherichia coli, Klebsiella pneumoniae, group B Streptococcus, Pseudomonas aeruginosa, Streptococcus pneumoniae, Staphylococcus aureus, Neisseria meningitidis, and Listeria monocytogenes. In preliminary tests with 11 spinal fluid specimens, 2 serum specimens, and 5 urine specimens from patients with culture-proved HI(b) meningitis, antigen was detected in all specimens in concentrations ranging from 1 to 7,000 ng/ml. Antigen was not detected in any of 62 clinical specimens which were culture negative for HI(b), including 11 spinal fluid specimens from patients with bacterial meningitis caused by microorganisms other than HI(b). The enzyme-linked immunosorbent assay technique described here is considerably simpler than radioimmunoassay and, based on concurrent tests with 14 positive clinical specimens, may be more sensitive than counterimmunoelectrophoresis. It seems, therefore, to hold considerable promise for clinical use in rapid detection of systemic HI(b) infections.