An interlaboratory validation study was conducted to establish the method performance characteristics of an immunoaffinity column (IAC) cleanup procedure followed by LC/MS for the determination of fumonisins B-1 (FB1) and B-2 (FB2) and combined FB1 + FB2 in corn. The test portion is extracted with acetonitrile methanol water (25 + 25 + 50). The extract is filtered, diluted with phosphate-buffered saline solution, and applied to an IAC. FB1 and FB2 are removed with methanol, followed by water, then directly determined by RPLC with MS detection using selected-ion monitoring of two characteristic ions in each case. Naturally contaminated corn samples were milled to a fine powder and mixed to produce three samples with target levels of combined FB1 + FB2 ranging from 350 to 4000 mu g/kg. Of 15 initially participating laboratories, two failed to report results and another did not follow the prescribed method. Thus, valid results were obtained from 12 participants located in 11 countries. Statistical analysis of the results produced RSDr values of 4.6-11.9, 1.9-12.6, and 1.4-11.5% for FB1, FB2, and combined FBI + FB2, respectively; the corresponding RSDR values were 19.8-23.8, 18.2-25.5, and 18.8-23.2%. The three concentration levels of combined FB1 + FB2 were 534, 1194, and 1954 mu g/kg. HorRat values for r and R were all <2.0, indicating that the method is suitable as a regulatory method for the enforcement of European Union limits for fumonisins in corn.