In the course of a screening program for antibiotics, a new variety of Streptomyces viridochromogenes (strain No. MCRL-0368) was found to produce a series of antibiotics primarily active against Gram-positive bacteria. Isolation of these antibiotics was attempted, and three main components (sulfomycin I, II and III) were obtained as amorphous powders. The present paper deals with their isolation, purification, physicochemical and biological properties. The antibiotics were primarily active against Gram-positive bacteria, anaerobic bacteria and mycoplasma, and the complex was named sulfomycin. Taxonomic studies on the sulfomycin-producing strain (Streptomyces viridochromogenes var. sulfomycini nov. var.) will be reported later. Based on physicochemical properties (e.g., amino acid constitution, elementary analysis) and biological activities, sulfomycins differ from previously reported sulfur-containing peptidic antibiotics (e.g., bryamycin, siomycin) and are concluded to be a series of new sulfur-containing peptidic antibiotics.