We studied the contact interactions of human peripheral blood mononuclear cells (MNC) and transformed mouse fibroblast cell line L929 (L-cells), namely their effects on morphological phenotype of L-cells. The morphological characteristics of the fibroblast, (cell area, nucleus-cytoplasmic ratio, cell spreading, cell shape) were estimated with the aid of fight scanning microscopy, followed by computer image analysis. Contact interaction between fibroblasts and MNC caused normalization of morphological phenotype of the fibroblasts (increase of cell area, shape factor, spreading and decrease of nucleus-cytoplasmic ratio). This phenomenon was revealed by analysis of both average morphological characteristics and population contents. Only a particular subpopulation of L-cells, but not the whole population, was shown to be normalized by the effects of MNC. For elucidation of responsible MNC population, which was capable of influencing on fibroblast morphological phenotype, we separated MNC in several cell types adherent cells, non-adherent cells, which were separated in E-rosetting cells, and non-E-rosetting cells. Only non-adherent and E-rosetting cells could normalize the morphological phenotype. E-rosetting population consisted of 85% of CD3(+) (T lymphocytes) and 15percnt; of CD56(+)/CD16(+) (natural killers). Supernatants of MNC, and MNC cocultured L-cells, obtained after 24 h incubation in the cell culture medium, were not able to normalize the morphological phenotype of the fibroblasts. They had small denormalizing effect on the fibroblasts (decrease of cell area, shape factor, spreading and increase of nucleus-cytoplasmic ratio). The results of this study indicate that the contact interaction between MNC and fibroblasts may normalize transformed fibroblast morphological phenotype. The dependence of fibroblast functional state on their morphological phenotype imply the presence of regulatory mechanism of contact interaction between fibroblasts and MNC, which determines many live processes in fibroblast.