A human neuroblastoma cell line (Paju) was resistant to 10 mM difluoromethylornithine, a concentration at which the growth of all mammalian cells normally stops. Ornithine decarboxylase from Paju was very resistant to inhibition by difluoromethylornithine in vitro (Ki = 10 microM compared to 0.5 microM for mouse kidney ornithine decarboxylase). After purification, apparently homogeneous Paju ornithine decarboxylase was inactivated with [3H]difluoromethylornithine and analyzed by polyacrylamide gel electrophoresis. Under denaturing conditions it was found to have an altered molecular structure, i.e. two nonidentical subunits of Mr = 55,000 and 60,000. Another unusual feature of Paju ornithine decarboxylase was its long half-life in vivo (T 1/2 = 8 h compared with 36 min in human HL-60 promyelocytic leukemia cells). The disappearance of immunoreactive protein was only slightly slower than the loss of catalytic activity. The long half-life of Paju ornithine decarboxylase was not shared by adenosylmethionine decarboxylase. Despite the altered structure of Paju ornithine decarboxylase, it was recognized by a specific antisera raised in rabbit against mouse kidney ornithine decarboxylase. The Paju karyotype did not contain double minute chromosomes or any large homogeneously staining region such as that seen in a mouse lymphoma cell mutant that is resistant to difluoromethylornithine and overproduces ornithine decarboxylase (McConlogue, L., and Coffino, P. (1983) J. Biol. Chem. 258, 12083-12086).