Salivary gland explant cultures from 3/16 (19%) of unfed nymphal Ixodes scapularis Say contained Lyme disease spirochetes, increasing to a maximum of 14/16 (88%) at 72 h of tick feeding. Homogenates of tick salivary glands did not produce infection in laboratory white mice unless harvested from ticks attached for greater than or equal to 60 h. Dispersal of spirochetes to the salivary glands appears to occur during the act of tick feeding, thus affecting the ability of ticks to transmit Borrelia burgdorferi.