Another study to be described here addressed what I chose to call ''ex-vivo infectivity.'' We undertook to maintain explants of intestinal mucosa in their native functional state outside the donor's body, so as to observe early events in enterovirus infection. A number of such studies had been done before ours, using explant cultures from swine, cattle, feline, and human fetal intestines (Bridger et al. 1978; Derbyshire and Collins 1971; Dolin et al. 1971 Dolin et al. , 1972 Hoshino and Scott 1980; Wyatt et al. 1973) ; we hoped to develop additional, useful information. Our first experiments were done with 2-3-mm square explants from the distal ileums of 22-30-cm fetal pigs (Jensen and Cliver 1984) . Five areas per Petri plate had been scratched to facilitate attachment; the prepared explants were placed mucosal-side-up in these areas and incubated at 37°C with MEM plus fetal calf serum and antibiotics. Porcine enterovirus 3 had been photosensitized by replication in the presence of neutral red (Wilson and Cooper 1965) : a strong pulse of visible light from a fluorescent lamp reduced the titer by 5 logs. The virus and the explants were held in the dark and handled by the light of a photographer's red safe light. A dissecting microscope showed apparently normal villous structure during the first 4 days, with significant deterioration thereafter; net yields of light-resistant progeny virus ranged from 1 to 3 logs by day 2 or 4.