Cytidine deaminase can cause the deamination of cytotoxic analogues of cytidine or rescue cells from the cytotoxicity of uracil analogues. Therefore, cytidine deaminase influences the cytotoxicity exerted by these compounds. We investigated the activity of this enzyme in situ in neuroblastoma cell-line cells. The in-situ cytidine deaminase activity in human neuroblastoma cell-line SK-N-SH cells appeared to be two-fold higher than in the human neuroblastoma N-myc amplified cell-line SK-N-BE(2)-C cells. The observed activity correlated with the presence of both alleles in SK-N-SH cells versus one allele of cytidine deaminase in SK-N-BE(2)-C cells. This observation may be exploited for the treatment of neuroblastoma patients having a tumour with a chromosome Ip deletion including the domain containing the gene encoding cytidine deaminase.