A new enzyme, N-methylhydantoin amidohydrolase, was highly purified from Pseudomonas putida 77: it catalyzes the hydrolysis of N-methylhydantoin to N-carbamoylsarcosine with the concomitant stoichiometric cleavage of ATP to ADP and orthophosphate. The enzyme absolutely requires ATP, MG2+ and K+ for the N-methylhydantoin hydrolysis. The rapid and complete degradation of N-methylhydantoin during the cultivation of P. putida 77, which rapidly degrades creatinine via only N-methylhydantoin and which shows high activities of the enzymes involved in creatinine degradation (Yamada et al. (1985) FEMS Microbiol. Lett. 30, 337-340), seems to be due to the continuous ATP-generation during cultivation.