Detection of nucleic acids from paraffin-embedded material by in situ hybridization with oligonucleotide probes is increasingly being used. To determine the effect of fixation on the preservation of DNA and mRNA, we studied 18 lymphoid tissues fixed in B5, formalin, OmniFix, ethanol, and Bouin's fixatives and embedded in paraffin by in situ hybridization, using biotinylated oligonucleotide poly d(T) probes and immunoglobulin light chain probes. Detection of DNA using the poly d(T) probe was most consistent and most intense in tissue fixed in formalin, followed by OmniFix and ethanol, with B5 and Bouin's fixatives yielding unsatisfactory results. Detection of mRNA, using the light chain probes, was most consistent and most intense with tissue fixed in formalin and Bouin's solution, followed by B5 fixative, with OmniFix and ethanol fixatives yielding unsatisfactory results. The results of mRNA detection using the poly d(T) probe were found not to correlate with mRNA content as determined by the light chain probes for several fixatives, possibly owing to selective degradation of portions of the mRNA molecule.