An analysis of the assay of 28 preparations of anti-D immunoglobulin using a radioisotope method carried out at 6-monthly intervals for 2-4.5 years showed an average fall in anti-D concentration ot 10.6% each year, with 99% confidence limits ot 6.8-14.7%. The fall in anti-D concentration after storage at 37°C for 1 month was less than 8%, the minimum change that could be detected. No significant change in physical characteristics of the immunoglobulin were detected. The error of a single estimate of anti-D by the radioisotope method (125I-labelled anti-IgG) used here was calculated to be such that the true value probably (p=0.95) lay between 66 and 150% of the estimated value.