Virginiamycin M.sub.1 having the Formula: ##STR1## and virginiamycin M.sub.1 analogs having the Formulae I-IV: ##STR2## Virginiamycin M.sub.1 and the analogs I-IV are antagonists of cholecystokinin (CCK) and gastrin. Cholecystokinin antagonists are useful as analgesics and in the treatment and prevention of disorders of the gastrointestinal, central nervous and appetite regulatory systems in animals, especially humans. Gastrin antagonists are useful in blocking the receptors for gastrin in humans and may function as agents for the treatment of ulcers, tumors or other gastrointestinal disorders. The compounds of Formulae I-IV are also antibiotics and are useful as antimicrobial agents in animals including man and are useful as food additives to promote feed utilization in animals. Virginiamycin M.sub.1 and the analogs of Formula I, III and IV are produced by the controlled aerobic fermentation of a strain of Streptomyces olivaceus, ATCC No. 53527, while the analog of Formula II is produced by chemical synthesis.