HsMCM2/BM28 is a member of the family of minichromosome maintenance (MCM) proteins, which play a critical role in DNA replication by helping to ensure that DNA is replicated once and only once per cell cycle. The association of HsMCM2 with DNA replication suggested to us that it might prove useful as a new marker for cell proliferation. To test this possibility, we employed immunohistochemistry and immunoblotting to study HsMCM2 expression in both normal human tissues and primary human tumors. We found that HsMCM2 was detectable by immunoblotting in 97% of the studied tumors but in only 27% of the corresponding normal tissues. In normal tissues, the immunoblot signal was weaker than in tumors. Immunohistochemistry revealed that in normal tissues HsMCM2 is present only in proliferating cell nuclei. In most cases, tumor cell nuclei produced a stronger HsMCM2 signal than normal proliferating cell nuclei. Comparative studies revealed that antibodies against HsMCM2 stained fewer nuclei than antibodies against proliferating cell nuclear antigen but usually more than antibodies against Ki-67 (another proliferation-related antigen). Thus, the correlations between proliferation and antigenic signal are different for these three proteins. These results indicate that HsMCM2 is, indeed, a novel marker for proliferating cells. Further studies are required to determine whether the fact that HsMCM2 has a different correlation with proliferation and elevated staining intensity in tumor nuclei (compared to nuclei in normal proliferating cells) will permit it to be a more useful diagnostic and prognostic marker than proliferating cell nuclear antigen and Ki-67.