The effect of formalin fixation on DNA and on polymerase chain reaction (PCR) amplification was investigated. Lambda phage DNA fixed in buffered formalin showed incomplete digestion on restriction endonuclease treatment. The resistance to restriction digestion was dependent on the temperature of fixation, but not affected by salt concentration of the fixative. Lambda phage DNA fixed in unbuffered formalin showed poor PCR amplification due to degradation of DNA during fixation. Lambda phage DNA fixed in buffered formalin evaded degradation and suited for template of amplification. Feasibility of formalin-fixed tissues as sources for PCR amplification was also investigated with primers producing 128 bp fragment of c-Ki-ras exon 2. Although DNA from tissues fixed for 3 months showed amplification, there was no amplification from tissues kept in unbuffered formalin for longer than 6 months.