INTRODUCTIONHistone methyltransferases catalyze the addition of one or more methyl groups to a specific lysine or arginine residue within histones. Currently, there is a great deal of interest in histone methyltransferases, because mutations and misregulation of the genes encoding these proteins have been linked to various cancers and other diseases. Many genes encoding putative histone methyltransferases have been identified in eukaryotes, but the proteins they encode have not been functionally characterized. This protocol describes an in vitro assay for histone methyltransferase activity that uses bacterial cell extracts in which expression of a methyltransferase of interest is induced. In many cases, purification of the enzyme is unnecessary, making this experiment ideal for pilot studies. Bacterial cell extract containing the methyltransferase of interest is incubated with S-adenosyl-L-[methyl-(3)H]-methionine and various histone substrates, many of which are commercially available. Incorporation of the methyl-(3)H can be measured easily by scintillation counting. The labeled substrate is visualized by SDS-polyacrylamide gel electrophoresis (PAGE) followed by fluorography. This allows the substrate specificity and activity of a histone methyltransferase of interest to be readily characterized.