Only 24 h at 37°C were allowed, to ensure that the explants were as normal as they looked. Retention and replication of the two viruses were compared in explants of absorptive and lymphoid mucosa from young and adult animals. Retention was limited, but favored CB5 in all cases and was greater in absorptive tissue than in lymphoid tissue; age differences were minimal. Replication of CB5 was also limited, but statistically significant and greatest in absorptive tissue from young animals and least in lymphoid tissue from young animals; yields from adult absorptive and lymphoid tissue were intermediate between these and approximately equal. We then focused more closely on the earliest interactions between these viruses and explants (Heinz and Cliver 1988) . Tritiated CB5 and PO1 were incubated with explants for 6 h at 6°C to measure and attachment or 1 h at 37°C to measure penetration, followed by liquid scintillation counting and autoradiography. Results at 6°C were anomalous, suggesting that this was not a valid temperature for measuring virus attachment to explant cultures. Retention at 37°C was apparently greater in adult absorptive and lymphoid tissue, but more specific in young lymphoid tissue, where the ratio of CB5:PO1 was 4.3. It was seen that only a small proportion of CB5 associated with the explants, and penetration was principally into the epithelial cells along the upper third of the villi (those at or approaching senescence) and/or the lamina propria. Virus that associated with enterocytes further down the villi or in the crypts apparently did not penetrate the cells. It seemed that, since the enterocytes into which the virus penetrated were nearing the end of their functional lives, this might to some extent explain the typical absence of diarrhea in enterovirus infections. The findings might be quite different if the experimental system comprised porcine ileal explants inoculated with porcine norovirus.