The International Collaborative Project on Micronucleus Frequency in Human Populations (HUMN) was organized to collect data on micronucleus (MN) frequencies in different human populations and different cell types. The test procedures considered by this project are assays using human lymphocytes (cytokinesis-block method), exfoliated epithelial cells, and other cell types. Data(including descriptions of the populations monitored, detailed test protocols, and test results)are being obtained from a large number of laboratories throughout the world and are being entered into a unified database. The information will be used to: 1)determine the extent of variation of “normal” values for different laboratories and the influence of other factors potentially affecting baseline MN frequency, e.g., age, gender and life-style; 2)provide information on the effect of experimental protocol variations on MN frequency measurements; 3)design and test optimal protocols for the different cell types; and 4) determine the extent to which MN frequency is a valid biomarker of ageing and risk for diseases such as cancer.