The catalytic activities of guinea pig liver transglutaminase toward glutamine-containing peptide derivatives of three series have been studied. These series include: (a) formylheptapeptides of the basic structure, HCO-GLY3-L-Gln-Gly3. A single L-leucine residue was systematically substituted for glycine at a different position in each peptide; (b) formyltripeptides of the basic structure, HCO-Gly-L-Gln-Gly. L-Leucine was substituted for glycine in each position and in both positions; (c) various N-acyl derivatives of the dipeptide, L-Gln-Gly. Comparison of the values of the kinetic constants for methylamine incorporation and for hydroxylamine incorporation with the peptide derivatives shows that the length of the peptide chain has a pronounced influence on catalysis, as does the position of the leucine residue in the longer chain peptide derivatives. The kcat/Km(app) values for each substrate calculated from data for methylamine incorporation and from those for hydroxylamine incorporation were found to be in good agreement. However, both the observed maximum velocity and the apparent Michaelis constant for each peptide derivative were significantly larger for hydroxylamine incorporation than for methylamine incorporation. Interpretation of these findings as evidence for a normal catalytic mechanism for each amine incorporation reaction and for the limiting nature of deacylation to methylamine is discussed. Two observations caution against such an interpretation. These are the significantly higher inhibitor constants found fo formylhexaglycine and for several other competitive inhibitors in the hydroxylamine incorporation reaction, and earlier findings of higher turnover values with hyroxylamine in cases were acylation appears to be limiting for methylamine incorporation. Methods of preparation, supporting analytical data and properties of the peptide intermediates, the peptides, and their derivatives used in this study are presented in the miniprint supplement immediately following this paper.