1. Intestinal structure, lactase (β-galactosidase; EC 18.104.22.168) activity and alkaline phosphatase activity have been determined in mouse jejunal and ileal tissues before and during infection with the intestinal parasite Nematospiroides dubius.
2. Oral infection with small numbers of N. dubius larvae caused villus height, crypt depth and enterocyte migration rate to increase in the mouse jejunum. None of these effects occurred in ileal tissue.
3. Lactase activity also increased in jejunal, but not ileal, tissue of infected mice. This increase was associated with a doubling of the rate at which activity appeared in the brush-border membrane of enterocytes during migration over the basal regions of jejunal villi. Alkaline phosphatase activity in jejunal tissue remained unchanged in infected mice.
4. Attention is drawn to the fact that this is the first occasion when crypt cell hyperplasia has been found to be positively correlated with an increase in lactase activity and a decrease in cytotoxic/suppressor T-cells. Further work is needed to establish the primary cause of these effects.