Neutral buffered formalin (NBF) (4% neutral buffered formaldehyde) has been advocated by most investigators as the primary fixative of choice for in situ hybridization (ISH), and specific anecdotal cautions interdicting the use of precipitating fixatives, which otherwise may offer certain advantages such as superior nuclear detail, are common. Few systematic studies addressing ISH fixation conditions have been published. We reasoned that heavy metals present in some precipitating fixatives may compromise duplex formation during ISH. Cell lines containing known viral gene content (CaSki, 200 to 600 human papilloma virus 16 copies/cell, and SiHa, 1 to 2 human papilloma virus 16 copies/cell) and two negative cell lines (K562 and MOLT 4) were expanded to >10(10) and pellets fixed in NBF, zinc formalin, B5, and Bouin's and Hollande's solutions, and subjected to DNA ISH using biotinylated genomic probes. Ten tissue biopsies fixed in both Hollande's and NBF solutions were also evaluated for human papilloma virus content using DNA ISH. Additionally, 17 cases of Hodgkin's disease fixed in B5 and formalin were compared for Epstein-Barr encoded RNA detection using RNA ISH with fluorescein isothiocyanate-labeled oligonucleotides. Catalyzed reporter deposition combined with Streptavidin-Nanogold staining and silver acetate autometallography (Catalyzed reporter deposition-Ng-autometallography ISH) and a conventional indirect alkaline phosphatase method were used for detection for both DNA and RNA. Contaminating heavy metals entrapped in fixed tissues were removed by two exposures to Lugol's iodine. Results for both DNA and RNA ISH comparing B5 and NBF fixatives were virtually identical. Hollande's, Bouin's, B5, and zinc formalin fixed tissue showed results indistinguishable from NBF fixed tissue in DNA ISH. Precipitating fixatives such as B5 and Hollande's solution may be used for DNA and RNA ISH under appropriate conditions.