A Borrelia garinii isolate (NE11H) was obtained from the hemolymph of infed Ixodes ricinus. NE11H expressed four major proteins of 33 kDa, 32 kDa, 23 kDa and 22 kDa. During in vitro culture, NE11H successively lost the expression of the 22 kDa and 23 kDa proteins and the NE11H variant (NE11Hp15) was not recognized by an immune serum specific for the OspC protein (anti-OspC IS). However, when reintroduced into tick midguts, NE11Hp15 spirochetes present in the midgut again reacted with anti-OspC IS. A clone derived from the wild type line, cNE11H, lacked the 22 kDa but not the 23 kDa protein. The 23 kDa protein of cNE11H was recognized by anti-OspC IS. In addition, the two descendant lines (NE11Hp15 and cNE11H) lost their capacity to induce clinical arthritis in SCID mice. When cNE11H was reintroduced into ticks and reisolated from various tick organs, most reisolates presented the same reaction with anti-OspC IS as cNE11H. Interestingly, two reisolates obtained from the tick midgut reexpressed large amounts of the 22 kDa protein which was recognized by anti-OspC IS and these two reisolates induced clinical arthritis in SCID mice. The results confirm that proteins of 22/23 kDa are differentially expressed during in vitro subcultures and in ticks, and show that proteins which are not detectable after in vitro culture may be reexpressed after reexposure of B. burgdorferi to its former environment in the tick. The data suggest that the pathogenicity of B. burgdorferi for mice might be influenced by environmental factors via differential expression of 22/23 kDa proteins.