The densities and distribution of innate immune system cells, NK cells and macrophages, was investigated between days 22-30 post coitus of very early pregnancy. Paraffin sections were labelled by the avidin-biotin complex-peroxidase method using monoclonal antibodies specific for CD45, CD56 and CD14. Positive cells were quantified and the results were analysed using ANOVA tests. It was found that ~40% of all cells were leukocytes and ~30% of the leukocytes were NK cells and ~22% were macrophages. The number of CD56+ cells increased as pregnancy progressed. CD56+ were observed cells close to the luminal epithelium and were especially noticeable around glands where they often occurred in small clusters. Decidual macrophages (CD14+) were distributed throughout the decidual stroma of early pregnancy and were observed in greatest numbers around the vessel walls. Their numbers have also increased as pregnancy progressed. The great abundance of these leukocytes, which are known as members of the innate immune system, in early pregnancy decidua suggests that these cells are important for fetal survival and pregnancy success. The relevance of the distribution of CD45+, CD56+ and CD14+ in decidua at the fourth week of pregnancy to the immunological barrier between mother and fetus is discussed.