The distribution of Cry1Ab toxin was detected in the leaves of genetically modified maize of genetic event MON 810 by enzyme-linked immunosorbent assay. Cry1Ab toxin contents in the leaves at reproductive (milk, R3) phenological stage were measured to be between 3,878 and 11,148 ng Cry1Ab toxin/g fresh weight. Toxin content was significantly lesser (significant difference (SD)&amp;#8201;=&amp;#8201;1,823 ng Cry1Ab toxin/g fresh leaf weight, p&amp;#8201;&lt;&amp;#8201;0.01) in leaves at the lowest leaf level, than at higher leaf levels, probably due to partial leaf necrotisation. A substantial (up to 22%) plant-to-plant variation in Cry1Ab contents in leaves was observed. When studying toxin distribution within the cross and longitudinal sections of single leaves, lesser variability was detected diagonally, with approximately 20% higher toxin concentrations at or near the leaf vein. More significant variability (SD&amp;#8201;=&amp;#8201;2,220 ng Cry1Ab toxin/g fresh leaf weight, p&amp;#8201;&lt;&amp;#8201;0.01) was seen lengthwise along the leaf, starting at 1,892 ng Cry1Ab toxin/g fresh weight at the sheath and rising to maximum concentration at the middle of the lamella. Cry1Ab toxin content may suffer significant (SD&amp;#8201;=&amp;#8201;2,230 ng Cry1Ab toxin/g fresh leaf weight, p&amp;#8201;&lt;&amp;#8201;0.01) decreases in the leaf due to necrotisation. The results indicate that the longitudinal dimension of the leaf has more significance for sampling purposes than the diagonal position