Most Borrelia burgdorferi strains have two major surface proteins, OspA and OspB. In the present study, we selected from a clonal population of infectious B. burgdorferi an OspB escape mutant, identified the genetic basis for this phenotype, and evaluated its functional activities. Selection with the anti-OspB antibody H614 was performed in vitro in medium and extended in vivo in scid mice. Mutants with a truncated OspB protein were selected at a frequency of 1 x 10(-5) to 3 x 10(-5). After no major rearrangements in DNA were detected, sequence analysis of the mutant's ospAB locus revealed a single base change in the consensus ribosomal binding sequence for ospB and a single nucleotide deletion in the ospB gene itself. The effect of these mutations was reduced expression of a truncated OspB protein. When functional abilities of the wild type and mutant were compared, the mutant had a threefold-lower capacity to penetrate a human endothelium umbilical vein cell monolayer. Infectivity of wild-type and mutant cells for scid mice was evaluated by culturing different organs, and the median infectious dose was calculated. The inoculum of mutant cells for infecting the mice was 30- to 300-fold higher than that of wild-type cells. This study shows that reduced size and expression of OspB are associated with lowered virulence of B. burgdorferi. Selection of mutants that to some degree remain infectious is one approach to defining the role of different surface proteins in the pathogenesis of Lyme disease.