Determination of parameters of the mitotic cycles in 5 human diploid embryonic strains and in 4 human strains from donor skin biopsies was conducted in termal room (37 degrees C). Diploid strains of postnatal origin have a longer duration both of the whole mitotic cycle (P less than 0.001) and of particular stages G1 (P less than 0.001), G2 (P less than 0.001) (duration of S-phase was not deviated) as well as lower values for the proliferative pool during the whole cultivation to the embryonic diploid strains.