In vitro dose-response curves of unstable chromosome aberrations in human lymphocytes have been obtained for 250 kV X-rays and cobalt-60gamma-radiation. The aberration yields have been fitted to the quadratic function Y = alphaD +betaD2, which is consistent with the single-track and two-track model for aberration formation. The values of the coefficients alpha and beta support the hypothesis that the dose-rate effect is limited to the D2 term. The main difference between the coefficients for X- and gamma-radiation is in the alpha values, indicating that X-rays are slightly more efficient, at lower doses, in producing two lesions with a single ionizing track. The lower limits of dose estimate, with 500 cells analysed, are 4 rad for X-rays and 10 rad gamma-radiation. Further evidence is presented confirming that, for cytogenetic dosimetry, in vitro dose-response curves should be prepared by irradiating whole blood maintained at 37 degrees C and prior to PHA stimulation. Curves were plotted showing the variation of the number of cells without aberrations with radiation dose and the shape of these curves were compared with those from human cell survival experiments.

discussed whether homologue affinity lies in the kinetochore or telomere; these two photographs suggest that either one could contain the necessary specificity for pairing.Thousands of chromosome preparations have been observed, but only one example of somatic pairing (Fig. 1) and four examples of the qua.drive.lent configuration (Fig. 2) have been seen. It is unfortunate that these particular chromosome spreads were not labelled, for autoradiographic patt,erns might tell something about the time during the lifo cycle of a cell when somatic crossingover and sister chromatid exchange occur.