An accurate plate diffusion bioassay for chloramphenicol is described, in which the fast-replicating Beneckea natriegens and 1.5% salt agar are used. Zones of inhibition were well defined after 3 h, and the limit of sensitivity of the method was around 2 μg/ml. The concurrent presence of gentamicin did not influence the assay. The assay is simple to carry out and duplicate assays can be performed with as little as 100 μg of capillary blood.