The authors have developed a test-system for detection of group A meningococcal polysaccharide, based on the sandwich erythro-immunoadsorption ultra-microtechnique with the use of the Terasaki plates as a solid-phase carrier. The system, equivalent to ELISA in its high sensitivity and specificity, is more rapid and less expensive, permitting the detection of 1 mg/ml of the antigen in 20 microliter of the tested liquid within an hour. The results of the study of CSF samples from 28 patients with meningococcal infection were in good correlation with the ELISA results. The new test-system is recommended for practical use as a routine technique for the specific diagnosis of meningococcal meningitis and for the control of the effectiveness of the treatment.