Since the first successful nuclear transfer (NT) experiments were carried out, various somatic cell types have been used as donor cells for production of cloned animals. In most experiments, fibroblasts are used since they only need to be isolated and cultivated. Recently, some researchers have shown that different cell cultures from different sources possess different capacities to support preimplantation development of NT embryos. The blastocyst rates obtained in our previous studies varied and were as high as 45% in relation to the number of reconstructed embryos. This led us to question whether the origin and culture conditions of the defined male and female fibroblast lines could be responsible for the differences in developmental potency. Taking all our results into consideration, we conclude that different fibroblast lines recovered from the same tissue and cultivated under equal culture conditions could produce dramatically different blastocyst rates. The influence of cell line itself is higher than the influence of passage number. The observed effects of cell cycle stage, chromosomal aberrations, and diminished vitality are important but not sufficient to discriminate well-qualified nuclear donor cells. We speculate that some epigenetically regulated deviations in the gene expression program are responsible for these phenomena. Explanation of the underlying mechanisms should contribute to better understanding of epigenetic reprogramming and may ultimately assist reprogramming in the laboratory.