The modification of the CAMP test for goup B streptococci involved substituting a paper disk impregnated with partially purified beta-hemolysin for the staphylococcal culture that was the source of beta-hemolysin in the original test. The disk is placed onto a sheep blood agar plate beside the streak of Streptococcus being tested. The plate is then incubated aerobically at 35 degrees C. A positive reaction consists of a lunar-shaped clear zone that appears within 24 h in the dark beta-hemolysin zone surrounding the disk. A double-blind study of 135 randomly coded streptococcal isolates showed complete agreement between the CAMP-disk test and the standard Lancefield precipitin test. All group B streptococci tested had positive reactions, and all strains tested from streptococcal groups A, C, D, and G were negative. The CAMP-disk test is a simple and convenient way to identify presumptively group B streptococci.