The Kresse strain of porcine parvovirus (PPV) was cloned into pUC19, and independent infectious clones were sequenced. The PPV Kresse and NADL-2 strains, which have different pathogenicities, shared an identical genomic organization and a high degree of sequence identity. Partial genomes (1.5 or 1.6 kb) of 15 field isolates were also amplified by PCR in regions with significant sequence differences between the laboratory strains. Five amino acid differences were consistently present within the VP1/VP2 coding region of the Kresse strain and virulent field isolates. A number of inconsistent point mutations were also found throughout the genomes of field isolates. In addition, among those with the vaccine amino acid profile, all but one isolate (IAF-3) contained a 127-bp noncoding direct repeat downstream of the capsid protein gene. The one exception was also the only vaccine-type PPV obtained from a mummified fetus. In order to identify genetic elements responsible for the distinct tropism (and possibly the pathology) of the Kresse strain, in vitro cell systems which differentiated the virulent from the vaccinal strains were established. Subsequently, chimeric infectious clones of the Kresse and NADL-2 strains were used to identify the allotropic determinant located in the VP1/VP2 region. The transfer of the BglII fragment of the Kresse genome, containing three amino acid differences, into the NADL-2 background, or the opposite construct, caused the phenotype of the target genome to revert to that of the parent strain of the BglII fragment. Prediction of the localization of amino acid differences on the basis of canine parvovirus capsid structure indicates that each is located on or near the outer surface of the virion. In particular, the position of one mutation (S-436-->P) maps by analogy to the threefold spike, the most accessible region of the capsid.