A beta peptides, the main protein components of Alzheimer's disease (AD) plaques, derive from a proteolytic cleavage of the amyloid precursor protein. Due to heterogeneous cleavage sites, a series of A beta peptides, including the major and widely studied species A beta 1-40 (A beta 40) and A beta 1-42 (A beta 42), are produced. In addition to the C-terminal heterogeneity of A beta peptides, significant amounts of N-terminal truncated (A beta 3-42) and pyroglutamate-modified amyloid-beta peptides (A beta pE3-42) have been identified in AD affected brains and shown to be more cytotoxic than unmodified A beta peptides. Little is known about the properties of their mixtures with A beta 42. Nuclear Magnetic Resonance spectroscopy is here employed to investigate the interaction of N-truncated peptides with A beta 42 at different molar ratios. We highlight the critical concentration of N-truncated forms influencing the aggregation kinetics of A beta 42. We provide evidence, at residue level, that the C-terminal region of A beta 42 is the locus of transient specific interactions with highly aggregation prone N-truncated alloforms.