Chromosome aberrations in human peripheral blood are recognized parameters of cellular damage and are used as indicators of exposure to ionizing radiation and certain chemicals. However, significant interlaboratory variability exists in the results reported from different laboratories. The primary objective of the present study was to examine problems associated with the identification and analysis of chromosome aberrations. Significant interlaboratory variability was found to exist in the analysis of human chromosome spreads for induced interphase aberrations, apparently owing in part to differences in the selection and rejection of spreads for scoring and also in the recognition and classification of various types of aberrations. These differences are reflected in the dose—response relationships for aberrations as well as for damaged spreads. For damaged spreads the scoring variability appears to be relatively small. It is inferred that factors additional to differences in scoring may play a significant role in the large variation in the reported dose—response relationships both for ionizing radiation and for chemically induced aberrations.