New antibiotics, dienomycins A, B and C were isolated from the culture filtrate of the strain MC67-C1 by a chemical screening method using Wood reagent. Dienomycins are derivatives of (4-phenylbutadienyl) pipecolin, Dienomycins A and B contain an alkoxyl group, and instead, C contains a hydroxyl group. Dienomycins are the first examples of microbial metabolites having piperidine and phenylbutadiene structures. Dienomycin A is active only against mycobacteria. Screening of culture filtrates of microorganisms for their antimicrobial activities has led to the discovery of various types of valuable compounds. If, however, culture filtrates are screened with chemical reactions, one would expect to find new compounds which might elucidate biosynthetic sequences. The authors have screened culture filtrates for their color reactions to the Wood reagent which is positive for purines, pyrimidines or imidazole derivatives. A compound found in this way also showed antimicrobial activities and was named dienomycin. The organism producing this compound belongs to actinomycetes. As shown by the chemical studies described in the next paper, dienomycin is not a purine, pyrimidine or imidazole derivative but a piperidine compound. Two analogues of this compound were also found in the same culture, and the screening method, production, isolation, physical and chemical properties of dienomycins are reported.