An indirect immunofluorescence test was used during 1982-1983 to identify antibodies to Lyme disease spirochetes in humans, white-footed mice, and raccoons. Serologic tests detected IgM or total Ig antibodies in serum samples from 67 persons. Onset of illness, as marked by erythema chronicum migrans (ECM), occurred mainly during July and August. The majority of the persons with Lyme disease lived in south central and southeastern Connecticut. Analyses also verified prior spirochetal infections in 29 of 323 (9 percent) white-footed mice and in three of 34 (9 percent) raccoons captured at sites with or without evidence of human infections. Results indicate potential for Lyme disease at numerous localities in Connecticut.