In the ICR strain, 155 gastrulas from 12 females were used for the cytogenetic analyses. About 18% of the gastrula cells from control ICR mice showed one or more chromosome anomalies (Table 9). Like in the CF1 strain, the anomalies were essentially of the chromatid-type, with a large excess of gaps. A few chromosome fragments and Robertsonian fusions (Fig. 2a) were, however, observed. Asimilar picture was observed in the first generation gastrulas of ICR females that had been X-irradiated with 0.2 or 0.4 Gy at the embryonic 1-cell stage (Fig. 2b and f). Like in the control group, several Robertsonian fusions (but no telomeric associations between more than two chromosomes) were observed in these two groups. The frequency of abnormal cells was significantly lower in the 0.2 Gy group, compared to the control groups (p = 0.022). However, the total numbers of chromatid and chromosome aberrations per 100 cells did not differ between the three groups (Table 9).