A b s t r a c t. In this paper we establish a new characterization of 4-valued modal algebras considered by A. Monteiro. In order to obtain this characterization we introduce a new class of algebras named generalized I-algebras. This class contains strictly the class of C-algebras defined by Y. Komori as an algebraic counterpart of the infinite-valued implicative Lukasiewicz propositional calculus. On the other hand, the relationship between I-algebras and commutative BACK-algebras, defined by S. Tanaka in 1975, allows us to say that in a certain sense G-algebras are also a generalization of these latter algebras.