A method and an apparatus have been developed for the determination of the susceptibility of pathogenic bacteria to antibiotics. The method has the same applicability as the accepted Bauer-Kirby method. The method is rapid, the procedure for the performance of the test is simple, the supplies and equipment for the performance of the test are inexpensive and finally the test gives easily measurable end-points. The correlation of the method to the accepted Bauer-Kirby method against clinical isolates of gram-positive bacteria was 90–100% for the antibiotics tested with the exception of neomycin which was 80%. With gram-negative bacteria, the correlation was 92–100% for the antibiotics tes