QM is a human cDNA originally isolated as a transcript elevated in a nontumorigenic Wilms' tumor microcell hybrid, relative to the tumorigenic parental cell line. The QM gene encodes a 24 kDa basic protein that peripherally associates with the ribosomes. Recently, the gene for this protein has also been shown in Saccharomyces cerevisiae to encode an essential 60S ribosomal subunit protein that is required for the joining of the 40S and 60S subunits. Since the association of QM with ribosomes can be disrupted with 1M NaCl, which has no effect on the association of core ribosomal proteins, indirect immunofluorescent cell staining was performed to colocalize the QM protein with the human large P-antigen, a core ribosomal protein of the 60S subunit, and to determine whether the assembly of the QM protein onto the 60S ribosomal subunit occurs in the nucleolus or in the cytoplasm. Our results reveal that QM co-localizes with the large P-antigen only to the cytoplasm where the rough endoplasmic reticulum is found and not to the nucleolus where ribosome assembly occurs. This finding suggests that the QM protein is most likely involved in a late step of the 60S subunit assembly and is added to the 60S ribosomal subunit in the cytoplasm and not in the nucleolus.