Isothermal bainitizing of high carbon steel is used to obtain exceptionally high mechanical characteristics together with near zero distortion during heat-treating. Such heat-treatments are often carried out at temperatures close to but above the martensite start temperature, where transformation kinetics are unfortunately very sluggish. The formation of a small amount of martensite prior to isothermal holding is known to accelerate transformation kinetics. However, the influence of the amount transformed remains to clarify, and the consequence on final properties have not been explored. In the following, the influence of partial martensite formation on the kinetics of bainite formation is investigated in 100Cr6 (AISI 52100) and similar grades. It is shown that, although the formation of martensite allows for shorter heat-treatment durations, it does not seem to affect the bainite kinetics, inasmuch as the bainite formation kinetics after formation of X% martensite are identical to that which would be expected if the first X% transformation had been obtained through isothermal bainitic transformation.