Eight cats were immunized with an avirulent strain of feline infectious peritonitis virus (FIPV)-UCD1, then challenge-exposed to a highly virulent cat passaged strain (FIPV-UCD8). Th1 and Th2 cytokine profiles in the peripheral blood mononuclear cells (PBMCs) were measured throughout in the experiment. No clinical signs of FIP were evident in the experimental cats after immunization. After challenge, the immunized cats demonstrated one of four clinical outcomes: (1) classical effusive FIP; (2) accelerated FIP; (3) non-effusive FIP, or (4) resistance to challenge. Only minor cytokine changes were observed following immunization, however, several cytokine changes occurred following challenge-exposure. The most noteworthy changes were in tumor necrosis factor-alpha (TNF- α) and interferon gamma (IFN- γ) levels. Our preliminary findings suggest that immunity against FIP is associated with TNF- α and IFN- γ response imbalance, with high TNF- α/low IFN- γ mRNA responses favouring disease and low TNF- α/high IFN- γ mRNA responses being indicative of immunity.