Absfruct. Conventional kittens, 12-27 weeks old, were inoculated with cell-cultured feline panleucopenia virus and killed sequentially between day 3 and day 24 after inoculation. All developed a non-fatal mild disease between days 2 and 9, characterized by lymphopenia, neutropenia, listlessness, depression and the development of neutralizing antibodies to the virus. Small intestinal bacterial counts were reduced during the period of maximal clinical disease, presumably a result of decreased food intake. There was involution of the thymus with marked depletion of lymphocytes between days 3 and 12. Depletion of lymphocytes also characterized the lesions in the lymph nodes between days 3 and 8. At the same time crypt lesions with spotty distribution were in the small intestinal and colonic mucosa. The changes were loss of crypt epithelial cells with compensatory attenuation of the remaining epithelium. Electron microscopically, the number and size of microvilli and secretory granules were reduced but there was no change indicating lethal cell injury. There were no virus particles. The findings point to an early and transient cellular damage by the virus. Intestinal alkaline phosphatase activity disappeared from the luminal surface of the attenuated crypt epithelial cells. Otherwise, intestinal alkaline and acid phosphatase activity were not altered in inoculated cats.