Tissue transglutaminase (tTG) protein begins to accumulate in apoptotic cells and its mRNA is expressed at the onset of apoptotic change. In the present study, we compared tTG expression with the atretic degree of mouse ovarian follicles. The whole-body gamma-irradiated mouse ovaries were collected and immunohistochemistry for tTG and in situ 3'-end labeling (TUNEL) was performed. Based on the identification of atretic follicles with hematoxylin-eosin and TUNEL immunostaining, tTG expression was evaluated and compared between normal (NF) and atretic follicles (AF). The expression of tTG was different among AF depending on the degree of atretic changes. There was a strong association of tTG expression with the follicular apoptotic changes. Among NF, 24% of follicles expressed tTG protein. This value, however, increased up to 66% in atretic follicles. The present results suggest that the follicular expression of tTG is closely related to the degree of follicle atresia. Therefore, the expression of tTG can be used as a useful marker for the identification of atretic follicles in the ovary.