To date, no standardized international guideline for the testing of chemicals for phototoxic potential has been accepted for regulatory purposes. In 1991, the European Commission (EC), represented initially by the Directorate General XI and later by ECVAM (the European Centre for the Validation of Alternative Methods) and COLIPA (the European Cosmetic, Toiletry and Perfumery Association), agreed to establish a joint EU/COLIPA programme on the development and validation of in vitro phototoxicity tests. The first phase (phase I, 1992-93) was designed as a prevalidation study, to identify in vitro test procedures and test protocols for a formal validation trial under blind conditions. In the second phase (phase II, 1994-95), the formal validation study, the most promising in vitro phototoxicity tests were validated with 30 carefully selected test chemicals in 11 laboratories in a blind trial. The 3T3 mouse fibroblast neutral red uptake phototoxicity test (3T3 NRU PT) was performed as a core test in nine laboratories, since it provided the best results in phase I of the study. The purpose of phase II was to confirm the reliability and relevance of the in vitro tests for predicting phototoxic effects and for identifying phototoxic chemicals. In phase II the phototoxic potential of test chemicals in the 3T3 NRU PT test was either assessed by determining the phototoxicity factor (PIF) by using a cut-off value of 5 as in phase I of the study, or by determining the mean photo effect (MPE) by using a cut-off value of 0.1, as recently proposed by Holzhütter (1997). Results obtained with both approaches in the 3T3 NRU PT test in phase II were reproducible in the nine laboratories, and the correlation between in vitro and in vivo data was very high. Therefore, ECVAM and COLIPA conclude from this formal validation trial under blind conditions that the 3T3 NRU PT test is a scientifically validated in vitro test which is ready to be considered for regulatory purposes for assessing the phototoxic potential of chemicals. A draft OECD Guideline for "In Vitro Phototoxicity Testing", incorporating the standard protocol of the 3T3 NRU PT test, will be submitted to the OECD test guidelines programme in due course.