Granulated lymphocytes with an unusual antigenic phenotype (CD56 + CD38 + CD2 CD3 - CD16 -) form a substantial proportion of leukocytes in human endometrial stroma. The purpose of this study was to examine morphological and anti- genie heterogeneity in endometrial granulated lymphocytes (eGL) in imprint preparations, paraffin-embedded sections and frozen sections. eGL in decidual imprints showed varia tions in cell size, nuclear size, shape and chromatin content and the number and size of cytoplasmic granules. eGL were detected in paraffin-embedded sections using phloxine tartrazine, alcian blue and toluidine blue stains. There was no difference in the number of eGL among the three stains but the granules appeared smaller and more regular when stained with toluidine blue. The proportion of stromal cells which were leukocytes increased from 8.2% in proliferative endometrium to 31.7% in early pregnancy decidua. The number of CD56+ and CD38+ cells increased in late secretory endometrium; CD56+ cells formed >75% of the leukocytes in first trimester decidua. The increased number of CD2 + cells in decidua was not comparable with CD56 + and CD38+ cells suggesting that a lower proportion of CD56 + cells in first trimester decidua coexpress CD2, an observation which was supported by double labelling studies. eGL therefore show morphological and antigemc hetero geneity and the study of granulated lymphocytes in patho logical endometrium and decidua will require careful phenotypic and morphological analysis of accurately dated samples.