Thiostrepton is well known as an antibiotic which inhibits protein synthesis. Recently, Thompson and his colleagues discovered that this compound induced the expression of many genes of unknown function in Streptomyces lividans. The tipA gene which encodes one of these proteins was cloned and sequenced. The tipA promoter (ptipA) was inserted in a promoter probe vector (pIJ486) so that it controlled the expression of a kanamycin resistance gene (pAK114). Although S. lividans (pAK114) was sensitive to kanamycin in the absence of thiostrepton, it became highly resistant to the antibiotic in the presence of very small amounts of thiostrepton (< 10~9m). Antibiotics structurally related to thiostrepton such as nosiheptide also induced ptipA, however, unrelated antibiotics did not. Here we report the use of this specific and sensitive assay to screen for thiostrepton-like compounds. Paper disks saturated with screening broth were placed on a kanamycin containing (5 jug/ml) nutrient agar plate seeded with a spore suspension of S. lividans (pAK114). After incubation for 24 hours at 30°C, a zone of S. lividans (pAK114) growth around the disk was presumed to indicate the presence of a thiopeptide antibiotic. As a result of this screening, novel thiopeptides, promothiocins A and B were isolated from Streptomyces sp. SF2741. In this communication, we report the fermentation, isolation and structure elucidation of these two compounds.