During the 1966–1971 period, an international comparison was made of measurements of the luminance factor and reflectance of pressed tablets of MgO and BaSO₄ and of MS-14 opal glasses. Twelve laboratories from eleven countries took part, many of them national standardizing laboratories. Variations in the manufacture of the pressed tablets and the effect of cleaning the opal glasses limited the precision of the comparison to some extent, but some useful information emerged. The interlaboratory variation was ±0.3% for the pressed tablets and ±0.6% for the opal glasses. Samples with improved properties should be developed for future intercomparisons of this type.