In a later study, we paid closer attention to maintaining normal organization of the villous mucosa (Heinz et al. 1987) . Intestinal tissue was collected from female Yorkshire pigs, either 4-6 weeks or 9-11 months old and prepared in 4-cm 2 explants; histology was monitored by light microscopy and scanning and transmission electron microscopy. Comparison of various medium formulations led to the selection of CMRL-1066, supplemented with insulin and cortisone, which would maintain apparently normal villous organization for 48 h. Explants were inoculated with either coxsackievirus B5 (CB5), which is infectious for swine, or with PO1, which is not.