A clone containing a 6.4 kb Borrelia burgdorferi chromosomal DNA insert reacted only with sera from patients with Lyme disease and not with any normal human or rabbit sera. Restriction enzyme analysis indicated that this DNA fragment was located on the B. burgdorferi chromosomal map between rpoB and p22A; its direction of transcription was towards p22A. Sequence analysis suggests that LA006 encodes six proteins: three previously described immunodominant lipoproteins of the 39 kDa Bmp protein family, BmpA, BmpB and BmpC; a 51 kDa MgtE magnesium transporter protein; a 16 kDa protein kinase C inhibitor; and a 56 kDa protein with similarity to an uncharacterized Escherichia coli chromosomal open reading frame.