Evidence for a no-threshold concentration-effect relationship for many environmental carcinogens, and especially for asbestos fibres, has been substantiated. Thus, the risk for humans of low concentrations of fibres in the diet could be predicted if the absorption of fibres through the gut could be measured. In this study, the behaviour of ingested fibres (asbestos and attapulgite) has been investigated by the collection of lymph, as described by Bollman et al. (1948). Fibres were found to cross the gut wall and to circulate in the abdominal lymph at 2-24 hours after feeding. The kinetics of the fibre passage could be measured: 50% of the total load absorbed was found in the lymph within the first six hours. The greatest fibre passage was observed with long asbestos fibres, which reached 2.1 X 10(-4) of the ingested load. Selection of the longest fibres was observed when size distributions were compared in the lymph and in the diet. negative results were obtained consistently in controls and in some of the treated animals. Lymph collection was found to be the most appropriate method for detecting intestinal absorption.