For almost 25 years, the central government laboratory of The Netherlands has administered a continuous program for the quality control of dairy products. The analytical work is conducted by private laboratories but monitored by the government laboratory. The design and results obtained by this program are illustrated by the determination of water and milk solids-not-fat contents in butter, using data collected monthly over a period of 19 years. Under this strict system of control, the within-laboratory reproducibility (i.e., over an extended period) is almost equal to the between-laboratory reproducibility. Most of this within-laboratory variation is from month to month, indicating that in this case the time variability dominates the between-laboratory variability.