An international collaborative study involving 23 collaborators was conducted to test methods, improved over previous methods with respect to speed and solvent use, for the rapid determination and thin layer chromatographic (TLC) confirmation of aflatoxin M1 identity in dairy products. For the quantitative method, collaborators assayed samples of Couda and cheddar cheeses, powdered milk, and butter containing levels of M1 near the anticipated limit of determination. Statistical analysis of the study results indicated that the lower limit of determination and precision of this method were comparable to these parameters of methods previously approved for analysis for aflatoxin M1. A few collaborators found that M1 eluted early from cleanup columns with cheese and butter samples and that emulsions formed during powdered milk sample extraction. The reasons for these problems have been determined and remedies are provided. For the TLC confirmation of identity method, collaborators prepared trifluoroacetic acid derivatives of M1 on the plates after 2-dimensional development. Concentrations as low as 0.3 ng/g cheese and 1.0 ng/g powdered milk were determined in this study. As a result of this study, both methods have been adopted as official first action methods by the AOAC and as reference methods by IUPAC.