Effect of several kinds of metal ions on the chain-length distribution of reaction products of polyprenyl diphosphate synthases was investigated. In the presence of Co2+ or Mn2+ octaprenyl-, solanesyl- and decaprenyl diphosphate synthases gave a variety of polyprenyl products with the longest chains being shifted by one or two isoprene units longer than those of the products formed in the presence of Mg2+. Thus octaprenyl diphosphate synthase became to give solanesyl (C45) and decaprenyl (C50) diphosphates when Mg2+ is replaced with Co2+ or Mn2+. Similarly solanesyl diphosphate synthase produced C50- and C55- diphosphates in the presence of Co2+ or Mn2+, and decaprenyl diphosphate synthase gave C55- diphosphate as the longest product in the presence of Mn2+. More remarkable effects were observed with Mn2+ than with Co2+.