With the help of the AFP(RIA) kit, 2 new standards of 10 ng/ml and 5 ng/ml were ob-tained by the double-dilution method. Sensitivity tests demonstrated the minimum detectable le-vels of the kits to be 1~4ng/ml. All the AFP levels in the serum of 350 normal adults were un-der 5 ng/ml. With <20ng/ml and <5ng/ml as normal thresholds, the diagnostic efficacy of AFP (RIA) for PHC was determined in 677 cases of benign or malignant liver diseases, and serial low-level AFP measurements were carried out over a long period of time in 3 postoperated PHC patients. The results show that with 5ng/ml as the normal threshold, it is beneficial to early diagnosis of PHC and the monitoring of postoperative reccurence.