Pathogenic leptospires can be causative agents of reproductive problems in pigs. Cultures of uteri and kidneys from two pigs herds in New South Wales and Victoria (Australia) yielded five strains identified as Leptospira on morphological and cultural grounds. Phenotypic characteristics (growth at 13 and 30 degrees C, growth in the presence of 8-azaguanine) were intermediate between those of pathogenic and saprophytic leptospires. No cross-agglutination was observed with reference antisera representing the 24 pathogenic serogroups and the main saprophytic ones. Antiserum against one of the strains did not agglutinate reference stains representative of any serogroup. This provided evidence of a new serovar, designated hurstbridge. Genomic characterization of the five strains was achieved using five molecular approaches. Mapped restriction site polymorphisms in the rrs (16S rRNA) gene were not related to those of any reference strains. Arbitrarily primed PCR fingerprints suggested clonality of the five strains. The strains all showed an identical and unique PFGE profile. PCR, using primers specific for the rrs gene of pathologic leptospires, amplified corresponding sequences from the strains. DNA-DNA hybridization (and reciprocal experiments) using the S1 nucleas/TCA method was performed between one of the strains and the reference strains of Leptospira species. The homology ranged from 0 to 36% (the latter being was Leptospira inadai) thus satisfying the criterion of a new species, Leptospira fainei (type strain BUT 6T). Phylogenetic analysis of 16S rRNA sequence showed that L. fainei and L. inadai formed a clade separate from the previously recognized 'saprophyte' and 'pathogen' clades.