We report on an international proficiency test (PT) scheme for mercury (Hg) determination in soil, sediment, fish and human-hair samples (ILAE-Hg-02). For total-Hg determination, 74% of participants had a satisfactory performance (|z-score| ≤ 2), 8% had a questionable performance and 18% required action (|z-score| > 3). The best results were obtained for soil, while fish yielded the most-biased results, reflecting the analytical problems of quantifying Hg at low concentrations. Proficiency in the extraction of organometallic and available metal fractions was an important goal of the ILAE-Hg-02 and it was concluded that most laboratories are still not acquainted with the procedures for Hg fractionation; we therefore discuss their importance and highlight the need for more PT schemes dedicated to this matter, as they are an important tool to the standardization of procedures and for the development of adequate quality control/quality analysis protocols and certified reference materials.