Mallory's triple staining is a histochemical technique used mainly for analysing connective tissues and glands and other tissues. We have described the differences in the nuclear staining between eutopic and ectopic endometrium as well as endometrial hyperplasia and adenocarcinoma using the Mallory's method. The ultrastructural differences between eutopic and ectopic endometrium have been detected. In normal and hyperplastic endometrium the presence of stromal cell nuclei with an increased affinity to aniline blue has been observed. The affinity has disappeared after digestion of tissues with RNase. In cases of endometriosis, independently of cell types, the nuclei have shown affinity to orange G. Similar effects in adenocarcinoma have been noted. The ultrastructural studies have shown that in normal endometrium the stroma contained cells with euchromatic and low electron density cell nuclei. In endometriosis heterochromatic cell nuclei present both in the stroma and within glands have been detected. The results indicate that the Mallory's technique may be a useful tool for recognizing the differences between eutopic and ectopic endometrium. The affinity for aniline blue in normal and hyperplastic endometrium occurs most likely due to increased RNA synthesis. Based on Mallory's staining a similarity between hyperplasia and unchanged endometrium in contrast to similar results of the staining obtained in cases of adenocarcinoma and endometriosis may be suggested.