Using preembedding immunocytochemistry at light microscopic level, we found that, although monoclonal antibody HNK-1 selectively outlined a subpopulation of GABAergic neurons containing a specific calcium-binding protein parvalbumin (PV) in the adult mouse cerebral cortex, the proportion of HNK-1-positive cells to PV-containing cells showed prominent regional difference. In the parietal cortex, approximately 50% of PV-positive cells were HNK-1-positive whereas only approximately 10% of PV-positive cells were HNK-1-immunoreactive in the occipital and temporal cortices. There were also prominent differences in this proportion among allocortical areas. These observations indicate that the cellular composition of chemically defined subpopulations of GABAergic neurons are different from area to area in mouse cerebral cortex.