The tissue-specific expression and seasonal abundance of Cry1Ab protein were determined in transgenic maize plants (Mon810, variety 'Novelis') from two field trials located near Bonn and Halle, Germany. A total of 1085 samples were analysed by using Double Antiserum-Enzyme Linked Immunosorbent Assay (DAS-ELISA). The Cry1Ab contents of various plant tissues (root, stem, upper leaf, lower leaf, anther, pollen and kernel) were determined at four different growth stages (BBCH19, BBCH30, BBCH61 and BBCH83) collected in 2001, 2002 and 2003. Mon810 showed the highest Cry1Ab contents in the leaves (5.5 - 6.4 µg g fresh weight &lsqb;fw&rsqb;) at BBCH83, whereas the lowest Cry1Ab contents were detected in the pollen (1 - 97 ng g fw). Cry1Ab content of residual root stocks collected in the field nine months after harvest was 15 - 17 ng g fw. This demonstrated that the Cry1Ab concentration in residual root stocks was reduced to about one-hundredth of the fresh roots. The monitoring of Cry1Ab expression showed that the Cry1Ab contents varied strongly between different plant individuals.