Sera of 1,333 African, Caucasian, Chinese and Malay males over 15 years from four countries were randomly assayed for alpha-fetoprotein (AFP) by the radioimmunoassay. The average AFP level was 2.97 IU/ml (+ 2 SD = 8.78, -2 SD = 1.00) which can be compared with previously published figures ranging from 2.3 to 1.2 ng/ml. Average values increased with age and the upper limit (mean + 2 SD) in the over 64 years age group was close to 17 IU/ml. The use of international units should facilitate the comparison of results between laboratories. The present data can help in determining whether an AFP value is "normal" and when AFP levels of certain follow-up patients revert to normal.