This invention concerns new biocidal compounds of formula (I) wherein R is H or CH.sub.3. Of particular interest are two isomers that are named malayamycin A and desmethylmalayamycin A. These isomers, which may be prepared by growing under controlled conditions a previously unknown strain of micro-organism from the species Streptomyces malaysiensis, are characterised by nmr and mass spectroscopic data. They have biocidal activity, including anti-fungal, anti-viral and anti-cancer activity, and are of special interest for use in agriculture, horticulture, animal and human health.