The "all or nothing" principle in the colony survival test has to be given up. By reproductive death of mammalian cells we can conclude that colonies with two and more cells in the colony survival must be considered. An arbitrary drawn low limit of cell numbers can give misleading results. 2. The colony-size spectrometry registers the reproductive lethal cell damages. These cell damages could be detected qualitatively as well as quantitatively. The sensitivity of this method is greater on low and middle X-ray dose as compared with the colony-survival test. 3. Sublethal damages with tendency of recovery as produced by the split-dose effect are not observed by colony-size-spectrometry. Therefore, this damage is not due to chromosomal X-ray-damages. 4. The combination of physical and radiogenic damages as well as the combination of radiogenic and pharmacological damages and lethal reproductive cell damages also can be seen. The colony-size-spectra observed show that only additive but no potentiating effects occur. 5. There is a linear relationship between MKD and X-ray dose. This relationship becomes asymptotic using cytostatics as in the case of Bleomycin and ICRF 159 in a dose range beyond approximately 2 mg/kg.
