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In 1977, the Commission of the European Communities initiated an intercomparison study to determine the single administration oral LD50 value in rats of each of 5 chemicals with the aims of comparing experimentation technologies, determining the degree of variation in the results and the various parameters used to establish the LD50 value, and establishing a common protocol for the determination of the LD50 value. Sixty-five laboratories in 8 countries took part in the first study. The significant variation in protocol may have led to the large interlaboratory variation observed in the results. Therefore, participating laboratories carried out a second study using a common protocol, preceded by a pilot study. A total of 100 laboratories in 13 countries participated. A majority of results of the second study are currently being analyzed, and the indications are that the interlaboratory variation has been significantly reduced.