The conversion of serine and tetrahydrofolate to glycine and 5,10 methylene tetrahydrofolate by serine hydroxymethyltransferase (SHMT, EC 2.1.2.1) is the major route for the provision of one-carbon units for biosynthetic reactions. SHMT cDNAs have been cloned from both rabbit and man, and a human mitochondrial SHMT gene sequence has recently been reported. We have isolated phage clones containing human genomic sequences homologous to cytosolic SHMT and have found these to contain a processed pseudogene (SHMT-ps1) with a 90% identity to cloned SHMT cDNAs. SHMT-ps1 contains 2335 nt that are homologous to SHMT but it has an additional leader sequence of 203 nt of unknown origin. The complete SHMT-ps1 sequence of 2538 nt is bounded by two 16 nt direct repeats that are characteristic of retroelement insertion sites. Two phage clones containing SHMT-ps1 have been mapped by fluorescence in situ hybridisation to 1p32.3-33. In addition, an SHMT CDNA clone hybridized to the same region and to 17p11.2-12. The latter is consistent with a previous localisation of the gene for cytosolic SHMT.