SUMMARY: The antibacterial activity of a strain of a Cephalosporium sp. was found to be due to a number of antibiotics. One group of these, named Cephalo-sporins P, is soluble in organic solvents and active only against Gram-positive bacteria. The P antibiotic(s) is produced in good yield in deep culture in a medium com-prising corn steep liquor and glucose at an initial pH of 6.3. A high aeration rate assisted by adequate stirring is essential. Submerged cultures 72 hr. old form convenient inocula. Under optimum conditions with 2 yo (v/v) of such an inoculum the maximum titre is reached after 47-50 hr. at 24", and thereafter falls. The production of a diffusible antibacterial substance by a species of Cephalo-sporium was reported by Brotzu (1948) in an apparently little-known paper which was brought to the notice of Sir Howard Florey by Dr C. 0. S. Blyth Brooke. Prof. Brotzu kindly provided a culture of the organism, which was one of a number showing antibacterial action which he has isolated from the sea near a sewage outfall off Sardinia. This locality had been chosen with the expectation that antagonistic organisms would be plentiful there in view of the ' self-purification ' known to occur in sewage. The organism, C. acremonium or a closely related species, grew at room temperature on a number of complex solid and liquid media preferably containing added carbohydrate (e.g. glucose, lactose or starch) and a 'titre ' of 1/10 to 1/15 (test organism not specified) was reached in 4 4 days. The addition of various amino-acids, extracts of yeast, corn germ, peas, etc., had little effect on the titre which was, however, about six times greater from one daughter strain selected from hundreds tested. Streaking tests on solid media showed marked inhibition of Salmonella