Feline infectious peritonitis (FIP) was experimentally induced in FIP virus (FIPV) antibody-positive and antibody-negative kittens after challenge exposure to live-virus aerosol. Seropositive kittens developed antiviral immunofluorescence and lesions more rapidly after challenge exposure than did seronegative kittens. In seropositive kittens, FIPV antigen was present in macrophages and large mononuclear cells in tracheobronchial lymph nodes, lungs and trachea on postchallenge-exposure day (PCD) 2; in liver and spleen on PCD 3; in kidneys and omentum on PCD 4; and subsequently in nasal turbinates, thoracic and abdominal lymph nodes, thymus, bone marrow, parotid salivary gland, eyes and brain. Initial antiviral immunofluorescence on PCD 2 coincided with the onset of viremia and vascular lesions. Systemic lesions characterized by perivascular necrotizing pyogranulomatous inflammation, phlebitis and thrombosis, fibrinous serositis, and generalized lymphoid necrosis developed on PCD 3 and 4. Coronavirus-like particles were observed by EM in cytoplasmic vacuoles or smooth endoplasmic reticulum of degenerating macrophages in inflammatory lesions. In seronegative kittens, antiviral immunofluorescence intracheobronchial lymph nodes was first detected on PCD 5; viremia occurred on PCD 6. Systemic necrotizing lesions, comparable with those observed in seropositive kittens on PCD 3 or 4, did not occur in seronegative kittens until PCD 13 or 16. In both groups of kittens, initial viral infection in regional lymphoreticular tissue was followed by viremia and infection of macrophages in reticuloendothelial organs (liver, spleen, lymph nodes) and perivascular locations. The accelerated onset of infection and lesions indicative of an Arthus-type reaction in challenge-exposed seropositive vs. seronegative kittens further supports the immune-mediated pathogenesis of FIP.