Gelsolin, a Ca2+-sensitive protein present in many mammalian cells, regulates actin filament length by multiple mechanisms. In tumor cells, increased amounts of F-actin without change in its total amount have been observed, suggesting a modification in the organisation of this protein. The authors have localized gelsolin, total actin, alpha-smooth muscle actin, and prekeratin in frozen sections of normal human breast and infiltrating duct carcinomas by immunohistochemistry. A positive staining for gelsolin was observed in normal epithelial and myoepithelial cells but not in stromal fibroblasts. In contrast, no staining for gelsolin was detectable in carcinomatous cells, with the exception of remaining myoepithelial cells; myofibroblasts of the stromal reaction displayed an intense positive reaction. The absence of gelsolin staining in the epithelial cells of breast carcinoma may reflect their dedifferentiation or proliferative and invasive activities. The appearance of gelsolin in stromal cells raises the question as to what is its function in this situation. The immunohistochemical detection of gelsolin may be a useful adjunct to the study of mammary gland epithelium malignant transformation.