The results of the first international proficiency test (GeoPT1) of geochemical laboratories are presented. A total of 49 laboratories contributed analytical data on G94, a sample of Threlkeld microgranite which had been collected at the International Conference, Geoanalysis 94. Assigned values were derived by robust statistical analysis of contributed data for 12 major elements and 39 trace elements, of which 3 (As, Pb and The) were provisional values. Z-scores were calculated, as required by the protocol using either ‘pure geochemistry’or ‘applied geochemistry’fitness for purpose criteria selected by contributing laboratories. An overview of the z-score results indicates that 28.7% of contributed data fell outside the -2<z<2 range indicating that unsuspected analytical discrepancies may be affecting the corresponding results.