A lyophilized human cord serum secondary reference standard (988 vials of 1 mL/vial) has been prepared at the Laboratory Centre for Disease Control (L.C.D.C.) in Ottawa to assist in the standardization of AFP determinations. HB 8Agnegative lots of cord serum were used to prepare the standard. The moisture content of the freeze-dried material was <0.5 percent. AFP mass calibration standards were prepared by direct weighing in duplicate of highly purified freeze-dried material. Two RIA calibration runs were conducted on the reference cord serum employing the WHO First International Standard for AFP (72/225) and L.C.D.C. mass standards. Parallelism of dose response curves within assays was demonstrated between reference cord serum and mass standards and between the latter and the WHO standard. The mean WHO kIU of AFP per vial of reference serum (± coefficient of variation) was 63.0 ± 6.4 and the mean mass of AFP determined as μ per vial was 80.7 ± 7.9. A small-scale (pilot) interlaboratory calibration of the reference cord serum in mass units resulted in heterogeneous AFP estimates which ranged from 54.4 to 80.7 μ/vial. Greater uniformity of the latter results was noted following conversion of the mass estimates to WHO international units using conversion factors provided by several laboratories.