Thirty retail samples of heat-processed corn foods, i.e. corn flakes, corn-based breakfast cereals, tortilla chips and corn chips, were analysed for fumonisins -- fumonisin B 1 (FB 1 ), fumonisin B 2 (FB 2 ) and hydrolysed FB 1 (HFB 1 ) -- as well as for protein- and total-bound FB 1 . Bound (hidden) fumonisins cannot be detected by conventional analysis. Improved methods for the determination of bound FB 1 were developed. The protein-bound FB 1 was extracted with 1% sodium dodecylsulfate (SDS) solution. The SDS, which interfered with high-performance liquid chromatography (HPLC) analysis, was then separated from protein-bound FB 1 by complexing with methylene blue followed by solvent extraction and hydrolysis with 2 N KOH. To measure total-bound FB 1 , the sample itself was hydrolysed with KOH. In both cases, clean-up was accomplished on an OASIS polymeric solid-phase extraction column and the bound fumonisins were determined by HPLC measurement of HFB 1 . Fourteen of 15 samples of corn flakes and other corn-based breakfast cereals analysed contained detectable levels of FB 1 with a mean in positive samples of 67 ng g −1 (13-237 ng g −1 ). Two samples also had detectable levels of FB 2 (21-23 ng g −1 ). Bound FB 1 was found in all samples; the mean protein-bound FB 1 measured was 58 ng g −1 (22-176 ng g −1 ) and the mean total-bound FB 1 measured was 106 ng g −1 (28-418 ng g −1 ), reported as FB 1 equivalents after correction for recoveries of HFB 1 . There was an average of about 1.3 times more FB 1 in the bound form compared with extractable FB 1 , and this was about twice as much as protein-bound FB 1. Seven of the 15 samples of alkali-processed corn-based foods, such as tortilla chips and corn chips, contained FB 1 and three contained HFB 1 with means in measurable positive samples of 78 (48-134) and 29 (13-47) ng g −1 , respectively. Five of these alkali-processed corn foods contained bound FB 1 ; the mean measurable protein-bound FB 1 was 42 ng g −1 (39-46 ng g −1 ) and the mean measurable total-bound FB 1 was 100 ng g −1 (54-209 ng g −1 ). HFB 1 derived from bound FB 1 in selected samples was confirmed by HPLC with mass spectrometry (MS).