Oncogenic transformation of C3H 10T1/2 cells was determined after exposure to graded doses of 4.3-MeV alpha particles LET = 101 keV/microns. The source of alpha particles was 244Cm and the irradiation was done in an irradiation chamber built for the purpose. Graded doses in the range of 0.2 to 300 cGy were studied with special emphasis on the low-dose region, with as many as seven points in the interval up to 10 cGy. The dose-effect relationship was a complex function. Transformation frequency increased with dose up to 2 cGy; it seemed to flatten at doses between 2 and 20 cGy but increased again at higher doses. A total of 21 cGy was delivered in a single dose or in 3 or 10 equal fractions at an interval of 1.5 h. An inverse dose-protraction effect of 1.4 was found with both fractionation schemes. Measurements of the mitotic index of the population immediately before the various fractions revealed a strong effect on the rate of cell division even after very low doses of radiation. Mitotic yield decreased markedly with the total dose delivered, and it was as low as 50% of the control value after 4.2 cGy and 20% after 14 cGy with both fractionation schemes.