Lectin cytochemistry was used to determine whether phase-specific changes could be detected in the smears of mouse vagina taken during the different phases of the estrous cycle. The strongest and most specific lectin reactivity was seen in the proestrus phase of the cycle. Several lectins reacted with mucinous cells typical of the proestrus phase and stained either diffusely the entire cytoplasm or only the cytoplasmic granules of desquamated cells. Nucleated squamous epithelial cells showed reactivity of the cell membrane or the entire cytoplasm in late metestrus and diestrus phase. Cornified squamous cells were essentially unreactive with the lectins tested in this study. Lectin cytochemistry is a reliable method for identification of mucinous cells in mouse vaginal smears and for establishing the cyclicity of the mouse estrous cycle.