Abstract: This invention relates to novel methods for the extraction of nucleic acid. In particular methods are described for isolating nucleic acid from a sample containing a complex biological mixture of nucleic acid and non-nucleic acids wherein the sample is combined with an extraction solution comprising a lactam and then the nucleic acid material is isolated from the resulting combined solution. The resulting combined solution is mixed and becomes biphasic and the nucleic acid material is isolated from the aqueous phase by precipitation with ethanol. The lactam is preferably about 5 to about 70% of the extraction solution and is most preferably 2-pyrrolidone, N-ethyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, N-dodecyl-2-pyrrolidone, N-methyl-2-pyrrolidone, N-hydroxyethyl-2-pyrrolidone, N-methyl-2-piperidone, 2-.epsilon.-caprolactam, N-methyl-2-caprolactam, 2-piperidone or N-(4-hydroxybenzyl)pyrrolidone. Methods for selectively isolating DNA, ribosomal RNA and plasmid DNA are also disclosed.