We have used nylon membranes spotted in duplicate with full-length polymerase chain reaction-generated products of each of the 4,290 predicted Escherichia coli K12 open reading frames (ORFs) to measure the gene expression profiles in otherwise isogenic integration host factor IHF(+) and IHF(-) strains. Our results demonstrate that random hexamer rather than 3' ORF-specific priming of cDNA probe synthesis is required for accurate measurement of gene expression levels in bacteria. This is explained by the fact that the currently available set of 4,290 unique 3' ORF-specific primers do not hybridize to each ORF with equal efficiency and by the fact that widely differing degradation rates (steady-state levels) are observed for the 25-base pair region of each message complementary to each ORF-specific primer. To evaluate the DNA microarray data reported here, we used a linear analysis of variance (ANOVA) model appropriate for our experimental design. These statistical methods allowed us to identify and appropriately correct for experimental variables that affect the reproducibility and accuracy of DNA microarray measurements and allowed us to determine the statistical significance of gene expression differences between our IHF(+) and IHF(-) strains. Our results demonstrate that small differences in gene expression levels can be accurately measured and that the significance of differential gene expression measurements cannot be assessed simply by the magnitude of the fold difference. Our statistical criteria, supported by excellent agreement between previously determined effects of IHF on gene expression and the results reported here, have allowed us to identify new genes regulated by IHF with a high degree of confidence.