In this study we describe the design and implementation of a novel low-density oligonucleotide microarray (the "Mytox-chip"). It consists of 24 mussel genes involving both normalizing elements and stress response related genes, each represented on the array with one or two different 50 mer oligonucleotide-probe reporters spotted in replicated samples on glass-activated slides. Target genes were selected on the basis of their potential involvement in mechanisms of pollutant and xenobiotic response. They are implicated in both basic and stress related cellular processes such as shock response, biotransformation and excretion, cell-cycle regulation, immune defense, drug metabolism, etc. The microarray was tested on mussels exposed to sublethal concentrations of mercury or a crude North Sea oil mixture. RNA samples were extracted from digestive glands of control and treated mussels for the synthesis of fluorescence labeled cDNAs to be used in dual color hybridizations. Transcription rates of two metallothionein iso-genes (mt10 and mt20), a p53-like gene and actin were quantitatively estimated also by real-time PCR to confirm microarray data. Significant alterations in the gene transcription patterns were seen in response to both treatments.