The presence and distribution of fibronectin and actin in breast fibroadenoma cells have been investigated in frozen sections and primocultures of the same samples, by means of indirect immunofluorescence techniques. In the tissue cells, epithelial cells were negative whereas myoepithelial cells were positive with the two antibodies. Moreover, fibronectin was mainly distributed in basal lamina of ducts and ductules whereas actin appeared to label stroma cells. In primocultures obtained from the same samples of fibroadenomas, the labelled patterns of fibronectin and actin were not really distinguishable between cells; the two protein distributions were quite usual. Our data on human breast fibroadenoma cells are in agreement with the concepts of cellular adaptation or selection in culture and emphasize the difficulty in correlating results of "in vitro" and "in vivo" studies.