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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 FB1. 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 2N KOH. To measure total-bound FB(1), the sample itself was hydrolysed with KOH. In both cases, cleanup 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 FB1; 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).