Participants in 19 national and international laboratories measured the activity concentration of samples of a solution of 137Cs by means of efficiency tracing. Samples of a 134Cs tracer solution, which had been standardized by ionization-chamber measurements in the framework of the international reference system, were distributed as well. The composition of the solutions, the preparation of sources and the counting equipment used are described briefly and an outline of the efficiency-tracing method is given. Results and uncertainties are presented in a table and a graph. The mean activity concentration, with its combined uncertainty (1σ), on the reference date was (604.1 ± 4.0) kBq g -1.