A serine hydroxymethyltransferase was purified to complete homogeneity from a serine-producing obligate methylotroph, Hyphomicrobium methylovorum GM2. The enzyme has a molecular mass of about 98 kDa and consists of two subunits of identical molecular mass. The holoenzyme exhibits absorption maxima at 280 nm, 340 nm and 415 nm in potassium phosphate buffer, pH 7.3, the last of which shifts with a change in pH (6.0-7.5) and contains 2 mol pyridoxal phosphate/mol enzyme. The holoenzyme is converted to the apoenzyme on incubation with phenylhydrazine and reconstituted on the addition of pyridoxal phosphate. The enzyme activity was inhibited on the addition of several sulfhydryl-modifying reagents and then recovered with 2-mercaptoethanol. One sulfhydryl group per subunit was found to be responsible for the activity. Isoelectric focusing showed that the enzyme has a pI of 5.6. The Km values for glycine, L-serine and DL-beta-phenylserine are 0.046 mM, 0.15 mM and 33 mM respectively.