We recovered a newly recognized spirochete from the blood, skin lesions (erythema chronicum migrans [ECM]), or cerebrospinal fluid of 3 of 56 patients with Lyme disease and from 21 of 110 nymphal or adult lxodes dammini ticks in Connecticut. These isolates and the original one from l. dammini appeared to have the same morphologic and immunologic features. In patients, specific IgM antibody titers usually reached a peak between the third and sixth week after the onset of disease; specific IgG antibody titers rose slowly and were generally highest months later when arthritis was present. Among 40 patients who had early disease only (ECM alone), 90 per cent had an elevated IgM titer (greater than or equal to 1:128) between the ECM phase and convalescence. Among 95 patients with later manifestations (involvement of the nervous system, heart, or joints), 94 per cent had elevated titers of IgG (greater than or equal to 1:128). In contrast, none of 80 control subjects had elevated IgG titers, and only three control patients with infectious mononucleosis had elevated IgM titers. We conclude that the I. dammini spirochete is the causative agent of Lyme disease.