Between 1969 and 1972 three quality control studies were set up to investigate the variation in results that was occurring between and within laboratories performing routine tests for the diagnosis of rubella infection. No attempt was made to standardize the test in these studies, and a wide range in titres of sera was reported. The aims of the present studies were:

(i) to investigate in greater detail whether results were more reproducible between laboratories if test sera were compared with control sera of known potency and the results given in international units of activity, and

(ii) to ascertain whether results between laboratories would be more reproducible if a standard test procedure was used.

Eleven laboratories participated in testing 38 sera on three separate occasions by a prescribed standard technique and by that used routinely in each laboratory. Eight of the 38 sera consisted of four pairs of duplicate samples.

Analysis of results of the study showed that the reproducibility between laboratories was substantially improved when the test sera were compared with a control serum of known potency and when a standard test procedure was used.

Variation in results between laboratories was least when a control serum of low rather than high potency was used. Variation within laboratories can be reduced by increasing the number of times the control and test sera are tested.

Since the rubella antibody content of the British Standard anti-rubella serum is expressed in international units, the potency of the control and results of test sera should also be expressed in such units.