Characteristic viral-type particles were seen in liver of kittens experimentally infected with the feline infectious peritonitis (FIP) agent. The particles were from 70 to 75 nm in diameter, with a central doughnut-shaped nucleoid 50 to 55 nm in diameter; numerous spikelike projections extended from their envelopes. Similar particles were seen by electron microscopy in peritoneal cell cultures derived from the peritoneal exudate of experimentally infected kittens, and viral antigens were identified in these cells by immunofluorescence. Cells and supernatant fluids from cultures containing these particles produced FIP when injected into the peritoneal cavity of kittens. The FIP agent is heat sensitive, ether labile, and relatively phenol resistant and is inactivated within 24 hours at room temperature. The FIP agent is inactivated by recommended viricidal concentrations of chlorhexidine and benzlkonium chloride.