The objectives of this study were to determine the factors that cause differences in the improvements of gel strength and epsilon(gamma-glutamyl)lysine (G-L) content in chicken and beef (Japanese black cattle) myofibrillar proteins after adding microbial transglutaminase (MTG). As the amount of MTG added increased, the breaking strength increased progressively (p < 0.01) in chicken and beef samples, with the exception of chicken samples treated at 40 degrees C. The values of elasticity in the chicken samples were lower than those of the beef samples (p < 0.01). Surprisingly, the elasticity level, epsilon(gamma-glutamyl)lysine contents and myosin heavy chain (MHC) band sizes of chicken and beef at all levels of MTG were significantly different (p < 0.01). The results of this study suggest that MTG activity was affected by MTG inhibitors; that MTG develops the texture of myofibrils differently in different species. However, the activity is limited and inconstant among meat proteins, as suggested by the data collected from the chicken samples. As a result, when the transferable amino acid residues are depleted (cross-linked) by MTG activity, the function of MTG will be insignificant. The correlation between MTG and different sources of meat protein is quite unstable but it is strong, which was observed when chicken and beef responded differently to MTG because their chemical and physiological properties were different. The remarkable rate of formation of cross-linked proteins and the discrepancy between the expected and observed amount of dipeptide raises the possibility that there are enzymes capable of reversing the reaction induced by transglutaminase in chicken and beef myofibrils. In summary, our results suggest that access of MTG to chicken and beef myofibrils is different because it depends on physiological (muscles and their fibre types), biological (substrates) and biochemical (inhibitors and amino acids) variables. (C) 2008 Elsevier Ltd. All rights reserved.