So far, only circumstantial evidence has indicated that aflatoxin is involved in the aetiology of liver cancer1, but we have found that it is possible to investigate this relationship by measuring the urinary excretion of aflatoxin M1, which is the hydroxylated product of B1. The work was carried out in the Philippines and was suggested by aflatoxin analyses of foods purchased locally. We wish to make clear, however, that the data in this report do not support or deny the contention that aflatoxin may be responsible for primary liver cancer.