The present proposal outlines phase II of a three part long term study involving feline enteric coronavirus (FECV) immunity. The goal is to create two small breeding colonies of cats selected specifically for either extreme resistance or extreme susceptibility to the virus. These colonies will serve as a basis for future studies on determining the genetic basis for resistance/susceptibility and to ultimately apply genetic selection for the control of feline infectious peritonitis (FIP) in pedigreed cats. FECV is not a significant pathogen by itself. However, mutations in the small envelope (3c) gene result in a novel virus with tropism for macrophages rather than gut epithelial cells. This novel virus causes a highly fatal disease known as feline infectious peritonitis. The FECV® FIP virus (FIPV) mutation occurs in about 5-10% of FECV infected cats, and one half or more of animals exposed to this mutant virus will fail to mount protective immunity and die. FECV immunity in most cats is either non-existent (about 10-20% of cats become persistent shedders) or short-lived (70-80% of cats undergo constant reinfections). Only 10-20% of cats develop strong immunity. Genetic factors are the best explanation for these differences in infection outcome. This phase II study is to determine whether the resistance/susceptibility to FECV infection is under genetic control. Phase I infection studies yielded a breeding colony of 7 animals. Kittens produced by these breedings, and not naturally infected with FECV, will be infected at 16 weeks of age with FECV and fecal FECV shedding monitored for 8 months. If heritability can be demonstrated, phase II studies will be extended to preliminary studies on a genetic basis of resistance and susceptibility. Phase III would be to apply specific genetic tests to detect resistance pedigreed cats and to see if FIP can be controlled by selective breeding.