Diamond-like carbon (DLC) coatings are deposited on glass substrates by plasma decomposition of gaseous carbon precursors in a direct current discharge. In addition to the hydrocarbon gas, CaO-H2O vapor is supplied to the continuously evacuated vacuum chamber via two mass flow controllers and a needle valve for the CaO-H2O inlet. While maintaining the same deposition time and relative partial pressure of the hydrocarbon, the relative partial pressure of the CaO-H2O vapor was modified. The first information about the biocompatibility was gained by cell experiments with L929 mouse fibroblasts, sessile drop tests, scanning electron and atomic force microscopy, and correlated to the microstructure of the coatings. While mouse fibroblasts of the type L929 attach and grow on unmodified DLC coatings synthesized by the decomposition of hydrocarbon, the addition of CaO-H2O into the precursor gas improves the coatings biological acceptance by the cells.