The effect of rotavirus and enterotoxigenic Escherichia coli, administered in different sequences, on alkaline and acid phosphatase, leucinaminopeptidase, beta-galactosidase, and succinicdehydrogenase of the intestinal mucosa of cesarian-derived, colostrum-deprived calves was investigated. Decrease in enzyme activity was most prominent in dual infections; it also occurred in parts of the small intestine in monoinfected animals. Increases in enzyme activity involved totally either one or all tissue compartments (crypt, basal villus area, villus tips). Increased activity was present in enteric mucosae that were either not affected or were only slightly affected by rotavirus or enterotoxigenic E. coli. We interpret the increase in enzyme activity as an adaptation of the enteric mucosa to maintain the absorptive function.