To obtain the subject a new antimicrobally active substance produced by Acinetobacter calcoaceticus adhered to sponge algae and useful as a medicine for humans, animals or fishes, and as agrochemical, food preservative or the like.

This new substance is obtained by the following procedure: sponge algae (Scientific name: Ceratodictyon spongiosum Zanardini) is washed with seawater and a phosphate buffer solution followed by addition of a phosphate buffer solution and then crushing the sponge algae; the resulting mixed liquid is centrifuged, the Acinetobacter calcoaceticus strain is isolated from the resulting supernatant and then cultured; the resulting supernatant afforded by centrifuging the culture solution is subjected to extraction with ethyl acetate, and the resulting extract is purified by column chromatography to obtain the objective new substance as high-purity extract which has high activity to suppress growth of Gram-positive bacteria (e.g. staphylococcus, streptococcus), also having an activity against Gram-negative bacteria.