Vaccines have proven ineffective in protecting cats against either feline enteric coronavirus infection (FECV), or its most serious sequel – feline infectious peritonitis (FIP). However, preliminary studies indicate that genetic selection for resistance may be possible. Ten to 20% of FECV infected cats will develop strong immunity and no longer shed virus, while an equal proportion will become persistent fecal shedders. The remainder of cats have an intermediate disease course, i.e. alternating between infection, immunity, loss of immunity and reinfection. These patterns of infection and immunity are best explained by genetic differences in susceptibility/resistance. If so, genetic selection for resistance to FECV might be the most effective means to eliminating or minimizing FIP mortality. The goal of this study is to tap into the resources of a large specific pathogen free domestic cat-breeding colony on the U.C. Davis campus. Accurate records are kept of the matings and progeny of cats in the colony. In a pilot study, 7 adult cats from this colony were experimentally exposed to FECV and their pattern of fecal shedding followed over 3-1/2 months. One of 7 cats rapidly cleared the infection and remained immune (strong immunity), 4/7 cats underwent repeated cycles of reinfection (waxing and waning immunity), and 2/7 cats became persistent virus shedders (poor immunity). The goal of this phase I study is to create two cohorts of breeding cats, representing the extremes in immunity; 1) cats that develop strong immunity, and 2) cats that remain chronically infected. Cats in each colony will then be bred and kittens tested for their pattern of FECV immunity. Hopefully, progeny will eventually breed true for either resistance or susceptibility. If this can be achieved, future phase II and III studies will look at the genetic basis for resistance and susceptibility. Hopefully, genetic markers may some day be available to identify breeding cats that have excellent or poor FECV immunity.