Students perform a disk diffusion assay according to the protocol recommended by the Clinical and Laboratory Standards Institute (formerly National Committee for Clinical Laboratory Standards). They learn about the stringent guidelines, including media preparation, culture preparation, inoculation with a swab, proper placement of antibiotic disks, and incubation conditions, that are necessary for accurate interpretation of results. The protocol is modified to include a second, more concentrated, inoculum. Differences in the diameters of the zones of inhibition produced by the two inocula are compared with the t test. Each student in a group of four makes independent measurements of the zones of inhibition, and the compiled data for each group are examined for differences using the analysis of variance test (ANOVA). Enumeration data is used to check the accuracy of dilutions.