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C-mos is a cytoplasmic upstream activator of the mitogen-activating protein kinase pathway with serine-threonine kinase activity. It plays a well established and vital role in oocyte maturation by participating in metaphase II arrest and meiotic asymmetric division, but little is known about its function in somatic cells. Recently, we observed overexpressed c-mos in a portion of non-small cell lung carcinomas (NSCLCS). In particular, c-mos immunoreactivity was detected in tumor cell nuclei in addition to its expected cytoplasmic localization, and c-mos overexpression was associated with chromosomal instability among other findings. To verify our earlier observations and to clarify further the role of c-mos in NSCLCS, we examined its distribution by both light and electron microscopy. We detected c-mos in the cytoplasm and/or nucleus of a portion of tumor cells and fibroblasts. In particular, granular immunoreactivity was observed in the cytoplasm closely associated with the rough endoplasmic reticulum. Nuclear staining was confirmed and was often found near the nuclear membrane, as well as in some large multilobular, possibly aneuploid, nuclei. C-mos positivity was also found in the nuclei of tumor cells undergoing apoptosis. Furthermore, c-mos was detected in areas with diminished vascularization. It should be noted that nuclear staining was found at the ultrastructural level more extensively than at the light microscope study. This suggests a masking effect by the hematoxylin nuclear counterstain.