The expression of the gene encoding the HLA-DR associated invariant chain (In-gene) in human B lymphocytes was analyzed by determining the level of invariant chain, mRNA in peripheral blood and bone marrow cells of several patients affected by hematological malignancies. In B cell neoplasms representative of different stages of B lymphocyte differentiation, In-gene activation was an early event that may occur in pre-B cells before immunoglobulin gene transcripts are detectable. The highest level of invariant chain mRNA were observed at an intermediate maturation stage corresponding to sIg, Ia, and B1 positive peripheral blood lymphocytes. At the terminal stage of B lymphocyte differentiation, the In-gene was turned off. In leukemic cell populations, the pattern of temporal activation of the In-gene corresponded to the pattern of activation of the genes encoding the HLA-DR alpha and beta chain.