Gene expression profiles for large sets of genes can be generated simultaneously with cDNA microarrays. The quantitative accuracy of this assay requires that the immobilized probes be present in abundant, non-limiting amounts relative to the amount of labelled target. When this is true, the rates of hybridization for the cDNA targets assayed will follow pseudo first-order kinetics and be limited only by the relative concentrations of the probes themselves, providing linear increases of signal relative to input target. We have examined how the levels of PCR product in the printing solution and the amounts of labelled cDNA target in the hybridization change net detectability and ratio stability in our assays, and will present statistical evaluations of their effect on reliability.