A silver-staining technique revealed the core morphology of metaphase chromosomes of irradiated CHO cells with chromatid lesions (breaks, gaps). These cells were photographed before and after silver staining. As a rule, the core was not continuous in chromatid gaps, suggesting that the chromatid is broken in many so-called gaps. Ten cytogeneticists who were asked to classify chromatid gaps and breaks from photographs of chromosome lesions before silver core staining agreed in only 19 of 53 cases.