1 The minimum toxic dose of tetrachloro-dibenzo-p-dioxin (TCDD) for man has been calculated from monkey and human data to be 0.1 microgram kg-1. 2 The amount of TCDD which man takes in from his environment is about 1/2050 of that present in one square metre of his immediate environment. 3 The average concentration of TCDD present in Vietnam immediately after spraying Agent Orange was 8 microgram m-2. 4 A soldier directly sprayed would attain an internal body concentration of 7 X 10(-5) microgram kg-1 or 1/1750 of the minimum toxic dose; soldiers moving through previously sprayed areas would ingest much less. 5 These quantitative aspects indicate that the dioxin sprayed with Agent Orange in Vietnam cannot have caused systemic illnesses in Vietnam veterans or birth defects in their children.