Intravenous inoculation of four age groups of White Leghorn chicks with ALV-F42, a group A field strain of avian leukosis virus (ALV), indicated that persistent tolerant infection could be induced as late as 2 weeks post hatch, though most birds responded with neutralizing antibody. Contact infection by environmental exposure to ALV was 100% effective in newly hatched and 28-day-old chicks. All contact-infected birds responded immunologically after transient viremia. A follow-up of immune birds from these six groups demonstrated that active multiplication of ALV continued despite neutralizing antibody. Infectious virus was shed by oral and cloacal routes, as well as through vertical transmission by hens to their embryos. Up to 10(8) infectious units of virus/g of feces was shed by 12-day-old viremic birds, and to a lesser extent virus was also shed in saliva as measured by oral washing. The cycle of contact transmission was also evaluated by the assessment of the efficacy of four portals of entry, where exposed skin was most effective in permitting infection, followed by oral, nasal, and conjunctival routes.