Is anyone aware of the steps taken by hatcheries to prevent this disease? Could it be linked to the egg peritonitis problems some people experience with their hens? Control: Lymphoid leukosis appears to be controlled best by reduction and eventual eradication of the causative virus. Breeder flocks are evaluated for viral shedding by testing for viral antigens in the albumen of eggs with enzyme immunoassays or by biological assays for infectious virus. Eggs from shedder hens are discarded, so that progeny flocks typically have reduced levels of infection. If raised in small groups, infection-free flocks can be derived with relative ease. These control measures are applied only to primary breeder flocks. Voluntary programs to reduce viral infection have already reduced mortality from lymphoid leukosis and improved egg production in most layer strains; similar programs are underway in certain meat strains. Some breeders favor, and have virtually achieved, total eradication, while others favor a reduced level of viral infection. Some chickens have specific genetic resistance to infection with certain subgroups of virus. Although genetic cellular resistance will unlikely replace the need for reduction or eradication of the virus, the cellular receptor gene has recently been cloned, and quick molecular assays for viral susceptibility could be developed. Thus far, vaccination for tumor prevention has not been promising. However, strategies are under consideration to induce antibodies in breeders with recombinant vaccines lacking infectious avian leukosis virus to ensure protective maternal antibodies in progeny chicks.