.alpha.-Substituted ureido-benzylpenicillanic acids of the formula ##STR1## (wherein R.sup.1, X and Y are as defined in the description) and pharmaceutically acceptable salts thereof, and a bactericidal composition comprising the above penicillanic acid or the salt and a pharmaceutically acceptable carrier are disclosed. The acids and salts thereof have high antibacterial activities against Gram-positive and Gram-negative bacteria, particularly those of the genus Pseudomonas.