Keratinocytes immortalized by the human papillomaviruses (HPVs) vary in cell morphology, growth properties, resistance to inducers of terminal differentiation, and karyotype. To determine the contribution of the host cell genetic background and the HPV genes to these cellular alterations, we have generated and characterized 6 human keratinocyte lines from two different newborn foreskins (A and B) using either the full-length HPV18 genome or the isolated HPV18 E6/E7 genes. The growth properties of the immortalized lines were found to correlate with the complexity of HPV genes present in the transfected vector. Interestingly, cell lines established from foreskin A revealed common chromosomal alterations regardless of the HPV construct utilized for immortalization, and these karyotypic changes differed from those observed in cell lines established from foreskin B, which exhibited their own characteristic aneuploid profile. Thus, chromosomal alterations of HPV-immortalized cells are in part determined by the host genetic background.