Microvillar membrane vesicle preparations with varying content of aminopeptidase N were prepared from enterocytes of the pig small intestine. Postembedding immunogold labelling of aminopeptidase N was performed on these glutaraldehyde/paraformaldehyde-fixed, osmium tetroxide-treated and Epon-embedded microvillar membrane vesicles. The number of gold particles per micrometre microvillar membrane (labelling intensity) was calculated and compared to the corresponding enzymatic activity. A very close relationship was found between labelling intensity and aminopeptidase N activity, demonstrating that postembedding immunogold labelling can be used quantitatively.