1. Methanol extracts of the skin of Bombina bombina and Bombina variegata variegata, two European discoglossid frogs, contain an active tetradecapeptide, bombesin. Alytesin, a tetradecapeptide strictly related to bombesin is present in extracts of the skin of Alytes obstetricans, another European discoglossid frog. The American frog Rana pipiens, contains in its skin ranatensin, an endecapeptide related to bombesin and alytesin.2. Passage of crude skin extracts of Bombina through a column of alumina yields eluates which may be considered free of other peptide contaminants and are suitable for the isolation of bombesin in a pure form.3. Bombesin has a stimulant action on several preparations of intestinal, uterine and urinary tract smooth muscle. Sometimes the effect is easily repeatable and shows a fair proportionality to the dose, but at other times a prompt and intense tachyphylaxis is observed. Other smooth muscle preparations are poorly sensitive or insensitive to bombesin. The rat uterus, the kitten small intestine, the guinea-pig colon and the rat urinary bladder may be used for the quantitative bioassay of bombesin.4. Bombesin-like peptides may easily be distinguished from all other naturally occurring peptides by parallel assay. They constitute a new group of active peptides possessing a peculiar spectrum of activity.