As authorized by the World Health Organization 29th Expert Committee on Biological Standardization, the preparation of human prolactin in ampoules coded 75/504 has been established as the International Reference Preparation (IRP) of human prolactin for immunoassay. From the results of a collaborative study, to which 15 laboratories in nine countries contributed, with the agreement of the participants, the content of each ampoule is defined as 0.650 International Units (i.u.; 650 mi.u.) immunoassay. The results of this collaborative study show that the IRP is adequately stable and suitable for use as a standard for the determination of prolactin in human plasma and serum. Estimates of the prolactin content of human plasma and serum made in the various laboratories have been compared and show good agreement in ranking order, but only fair agreement in the numerical value of the estimates. Numerical agreement is poor between estimates of the human prolactin content of two samples identical except for coding; this shows the difficulty in achieving continuity of estimates when any laboratory calibrates a replacement standard.