SUMMARY:Optimum conditions for the use of night blue and victoria blue as lipolysis indicators in fat emulsion agar medium required a dye strenth of 1: 15,000 in a medium of pH 8·0 containing 5% fat dispersed by hand shaking. Pour plates should contain 6 ml. and streak plates 10 ml. of the medium in a standard Petri dish. Incubation should be for 5 days at 30°. The pinkish‐mauve medium with clear blue‐zoned lipolytic colonies gave the same results as butter fat agar without dye but treated with CuSo4, when tested with 962 pure cultures. The inhibitory powers of the dye were assessed and although strongly toxic in the aqueous phase to Gram‐positive bacteria, victoria blue appears to have none to slight inhibitory power in the fat agar medium: night blue suppressed growth to about the same extent as tributyrin. The lipolytic flora of butter and to some extent milk shows a remarkable dominance of micorcocci. Organisms lipolytic on fat agar media are able to produce appreciable acid in a fat emulsion in a liquid