The cooling method is of major importance in the making of large crystals of high quality, but there has been no tool to efficiently determine an optimum cooling rate for protein crystal growth. We previously developed a new temperature control tool, temperature-at-once (TAON), which could produce multi-temperatures on a single microplate by generating a temperature gradient. We have now added a new function to control the cooling rate in TAON with a personal computer. The improved TAON enables us to grow protein crystals at multiple cooling rates on a single microplate at a time. We can now search more efficiently for an optimum cooling rate for crystal growth. (c) 2006 Elsevier B.V. All rights reserved.