In uninfected mice, the percentage of IgA-containing cells labelled one day after a single injection of tritiated thymidine [( 3H] d Thd) was constant in the upper, middle and lower small intestine. In uninfected mice receiving an additional injection on the two preceding days there was about a threefold increase after the three injections indicating accumulation of labelled cells during the three day period. Five days after infection with Trichinella spiralis, the percentage of IgA cells labelled after a single injection of (3H)d Thd was increased at each site indicating increased division and, or, localisation of IgA cell precursors in the intestine. The numbers of cells labelled after three injections were reduced at this time, with no evidence of accumulation of further cells labelled from the two additional injections on previous days. By nine, 13 and 18 days after infection the numbers of IgA cells labelled after both single and triple injections were each increased, indicating increased division and, or, localisation of IgA precursors in the intestine with accumulation of labelled cells over the three day period resembling that in uninfected mice.