Plastic polymers can be combined with additives that modify physical properties and stability of the material. However, the biocompatibility of those additives is not well known. The objective of the study was to characterize the impact of zinc stearate-a common additive-through the development of a novel three-dimensional (3-D) in vitro platform with endometrial cells from domestic cats. Epithelial and stromal cells from adult uteri were isolated and cultured in medium supplemented with 3% Matrigel for two weeks in plastic tissue culture dishes that had been identified as polystyrene with and without zinc stearate by Raman, FTIR, and X-ray fluorescence spectroscopies. Three-dimensional cell structures that were obtained were measured and categorized by shape. Cell viability, proliferation, differentiation, organization, and apoptosis then were assessed by immuno-staining. Results indicated that zinc stearate did not affect 3-D endometrial cell structure morphology, viability, or cellular composition. This first study of a new in vitro platform will be useful for studies testing the influence of other additives, drugs, or exogenous hormones.