Let A and B be matrices of M n (C). We show that if exp(A) k exp(B) l = exp(kA + lB) for all integers k and l, then AB = BA. We also show that if exp(A) k exp(B) = exp(B) exp(A) k = exp(kA+B) for every positive integer k, then the pair (A, B) has property L of Motzkin and Taussky. As a consequence, if G is a subgroup of (M n (C), +) and M → exp(M ) is a homomorphism from G to (GL n (C), ×), then G consists of commuting matrices. If S is a subsemigroup of (M n (C), +) and M → exp(M ) is a homomorphism from S to (GL n (C), ×), then the linear subspace Span(S) of M n (C) has property L of Motzkin and Taussky.