The purpose of this guidance was primarily to assist sponsors as they conduct testing in support of the WPMN exploratory program. It therefore focuses on the kinds of tests that address the endpoints and the types of nanomaterials being tested under the sponsorship programme. It is recognised, however, that due to the wide variety of nanomaterials, it is difficult to develop advice applicable to all nanomaterials; accordingly, the performer of a study will have to exert some judgment on a case-by-case basis on the applicability of the recommendations given in this guidance to their particular material.