Two Salmonella typhimurium isolates were studied, one as a representative from a series of neonatal meningitis cases treated at an Istanbul teaching hospital, the other from a gastro-enteritis case seen at a different Istanbul hospital. Both isolates were resistant to extended-spectrum cephalosporins, as well as penicillins, aminoglycosides and chloramphenicol. Cephalosporin resistance depended on production of PER-1 beta-lactamase, which is an extended-spectrum class A enzyme that is only distantly related to TEM and SHV enzymes, and which was previously known only from Pseudomonas aeruginosa isolates. The PER-1 gene was carried by an 81-MDa plasmid, which also determined resistance to aminoglycosides and chloramphenicol. Although it was not self-transmissible to Escherichia coli, this element did transfer if mobilised with plasmid pUZ8. The two S. typhimurium isolates gave indistinguishable DNA restriction patterns and, in addition to their 81-MDa plasmid, also contained 52- and 2.8-MDa plasmids, the last of these encoded TEM-1 enzyme. The two isolates were identical in serotype, antibiogram and plasmid-profile but nevertheless differed in phage type, and, therefore, represented distinct strains. The emergence of cefotaxime and ceftriaxone resistance in salmonellae is disturbing, since these agents are preferred therapy for neonatal meningitis caused by members of the genus.