138 Ixodes persulcatus and 140 I. ricinus ticks were collected by flagging in the Moscow region of Russia in April-September 1995. Borrelia burgdorferi infection rates of 171 (73 I. persulcatus and 98 I. ricinus) flat ticks were ascertained by both a dark-field microscopy and an enzyme-linked immunosorbent assay (ELISA). The ELISA is based on the use of "capture" monoclonal antibodies (mABs) to Osp A L12-9B4 (2D8) in combination with rabbit antisera to Osp A and B. This mAB reacts with B. burgdorferi sensu stricto well as B. garinii, it has no reactivity with B. afzelii (isolate Iper3, Russia). Darkfield examination showed 11% infected ticks. The Osp A antigen positivity rate was 12%. 87% concordance between the two assays was recorded. In this study we investigated homogenates of 94 (33-I. persulcatus, 60-I. ricinus and 1 hymph Ixodes sp.) engorged ixodid ticks removed from humans for the presence of B. burgdorferi by the ELISA. Osp A of B. burgdorferi was found in 18 (19%) of the ticks. Osp A was also found in 57 of 107 (53%) ixodid ticks collected on vegetation in the Moscow region. This group of ticks was not tested by a dark-field microscopy as they died shortly after being collected.