The composition and distribution of immunocompetent cells in human decidua basalis at 6-8 weeks of pregnancy was studied immunonohistochemically under normal conditions (control group) and in infection with different mycoplasma species (microbial number> 104 CFU). In all the groups studied, monocytes were most numerous cells; large granular lymphocytes (phenotype: CD56++CD16-CD3), macrophages (CD68+) and immunonegative lymphocytes were observed in equal proportions; the least numerous populations were those of NK-cells (CD56+/-CD16+), plasmocytes and granulocytes. No statistically significant differences were found between the content of these cells in all the investigated groups. The data obtained suggest that mycoplasma infection even characterized by high microbial number, does not cause significant changes in the composition of decidua basalis cellular infiltrate.