The gastrointestinal absorption constant (fi) is a critical parameter in assessing systemic uptake following the ingestion of a radioactive material and in monitoring such intakes. This study addresses the latter, particularly for plutonium, and from environmental measurements derives an f, value of 4 X for the Marshallese population. The uncertainty associated with the methodology and measurements used in this fi value assessment is evaluated. This evaluation takes into account the results from 24-h urine samples and the particular lifestyle of the Marshallese. Plutonium intake resulting from soil consumption is a primary parameter in this evaluation; for this study, it was assumed to be 500 mg d-l. Thef, value determined here is consistent with the values in ICRP Publication 67 of 5~1 0 -~ for ages 1 to adult, and is the same as that suggested by the NRPB. Health Phys. 73(1):167-175; 1997