Proteasomes generate peptides from intracellular endogenous and viral proteins for presentation by MHC class I molecules. During viral infection, interferon-γ (IFN-γ) acts as a cytokine altering the catalytic specificity of proteasomes by inducing the synthesis of the three proteasome subunits, low molecular weight protein (LMP) 2, LMP7 and multicatalytic endopeptidase complex-like 1 (MECL1). LMP2 and LMP7 have been shown to favour the presentation of certain antigenic peptides. These subunits are constitutively expressed in cell lines related to the immune system and IFN-γ-inducible in other cell lines. Less is known about MECL1. To reveal the extent of constitutive and IFN-γ-induced expression of MECL1, we studied MECL1 in different cell lines by Northern and Western blotting. The two B cell lines IM9 and Reh showed high constitutive expression of MECL1, only slightly induced by IFN-γ stimulation. The B cell line Daudi and the monocyte cell line THP-1 expressed MECL1 constitutively at an intermediate level. The MECL1 protein level in the THP-1 cells increased markedly in response to IFN-γ. In cells unrelated to the immune system, a very low constitutive expression of MECL1 was detected, highly inducible by IFN-γ. These results indicate that, similar to LMP2 and LMP7, MECL1 is constitutively expressed at high levels only in certain cell lines and can be induced by IFN-γ in other cell lines. The differential expression of MECL1 may be of importance for which antigenic peptides are presented by different cells as well as by the same cells at different IFN-γ levels.