Human whole blood was irradiated with 220 keV X-rays at doses of 0-4.0 Gy. After incubation periods of 48, 60, 72, 84 and 96 h, lymphocytes were prepared without colcemid pretreatment according to 2 different methods, and micronuclei were scored. The crucial point of lymphocyte preparation was found to be the osmotic pressure of the hypotonic solution. Only a method that preserves the cytoplasm of lymphoblasts is suitable for a correct association of micronuclei with the main nucleus. Similar as for structural chromosome changes, now their intercellular distribution can be analysed. This is necessary for the derivation of appropriate statistical weights which have to be used for more reliable regression analyses. For 48 h, the data can be described by the linear model, for 84 and 96 h, by the linear-quadratic model. For 60 and 72 h no such definite conclusions can be drawn. For calibration purposes a standardized culture time cannot be recommended. Because the background frequency is high, the lymphocyte micronucleus assay system is not sensitive enough to detect a significant increase in the incidence of micronuclei after exposure to low doses (less than 0.3 Gy).