Ureidopenicillins exert antibacterial activity on many Gram negative bacteria and also some Gram positive bacteria Two acylureido penicillins called azlocillin and mezlocillin have been introduced recently into medical practice in many countries as broad spectrum antibiotics. In this communication we present the sensitivity of Gram negative bacteria isolated from urinary tract infection of pediatric age patients to azlocillin, mezlocillin, another semisynthetic penicillin called pipercillin and aminoglycoside antibiotic gentamicin by MIC and disk-diffusion methods. The results are discussed in the view of the use of new semisythetic penicillins in clinical practice.