The excretory–secretory (E–S) products of the parasitic nematodes Trichostrongylus colubriformis and Nematodirus battus were found to modify the in vitro proliferation of the tumorous colic HT29-D4 cell line of epithelial origin. A characteristic feature of these E–S products is the presence of a high level of acetylcholinesterase (AChE) activity, the biological significance of which remains unclear. To determine a possible role of AChE on cell growth, the enzyme was purified from E–S products using edrophonium chloride. Purity was confirmed by polyacrylamide gel electrophoresis, using silver and Karnovsky stains, before assessing its effects on cell proliferation. The purified AChE was incorporated at different concentrations in a culture medium of HT29-D4 cells. A mitogenic effect was shown for low concentrations (0·1–14 units). By contrast, an inhibitory effect was noted at high concentrations (35–1400 units). Furthermore, polyclonal antibodies were prepared and depletion of AChE in E–S products by immunoprecipitation or affinity chromatography resulted in a partial or total disappearance of the stimulatory effect of cell growth. Thus, the results from this in vitro study suggest a modulatory role for AChE secreted by nematode parasites on the proliferation of epithelial cells of the host.