Factors that affect the termination of isoprenoid chain elongation catalyzed by prenyltransferase were investigated. The chain-length distribution of reaction products of solanesyl diphosphate synthase [EC 2. 5. 1. 11] homogeneously purified from Micrococcus luteus changed dramatically according to the concentration of the complex formed between isopentenyl diphosphate and Mg²⁺ (IPP-Mg) in the reaction mixture. However, the concentration of the complex between farnesyl diphosphate and Mg²⁺ (FPP-Mg), the priming substrate for this synthase, did not affect the product distribution, provided that the concentration of IPP-Mg was maintained at a certain level. Thus, the level of IPP-Mg is decisive in affecting the chain length distribution of the products of the prenyltransferase reaction, and the Mg²⁺-dependent variability of product specificity so far observed can now be understood in terms of the effect of IPP-Mg concentration.