After the introduction of new soil test methods in the state of Sao Paulo in 1983, a sample exchange program was organized with the objective of encouraging uniform use of the methods and to improve the analytical quality of the results. The program started in 1984 with the laboratories that adopted the new methods and has increased almost linearly during the last years, attaining 50 laboratories from 6 states in 1992. Statistical criteria were adopted for the rejection of discrepant results. A system for ranking laboratories was developed based on error estimation due to inaccuracy and imprecision. Analytical difficulties and most common errors have been discussed during annual meetings of the program. All laboratories involved use the same methods and errors due to inaccuracy and imprecision were, in 1992, reduced respectively to 36% and 44% of the values of 1986. The program contributed to increase credibility of soil analysis in Sao Paulo state.