Immunoelectroosmophoresis (IEOP) or countercurrent immunoelectrophoresis has been used to develop a rapid procedure for grouping group A, B, C, D, G and H streptococci. The group-specific carbohydrates were extracted by conventional methods such as treatment with acid or formamide and compared with extraction by autoclaving and pronase treatment for evaluation of the convenience with which many samples from clinical isolates can be handled. Grouping was then performed with commercial antisera for these 6 groups and the specificity and sensitivity of the test was compared with those of gel-immunodiffusion (ID) experiments. Immunoprecipitates developed in IEOP after 15 to 30 min (15 V/cm, 4‡ C). IEOP was up to 20 times more sensitive than ID or capillary precipitin tests, which makes it a rapid and convenient method. Clinical isolates could be typed the same day by IEOP. The IEOP method was also suitable for handling many samples at one time, which makes it a useful method for a diagnostic laboratory.