Cells were isolated from post-radiation fibrosis biopsies of patients with recurrent breast carcinoma. These cells were identified as fibroblasts and compared with fibroblasts from normal breast tissues for their proliferative activities, chromosome number and for the presence of various components of the extracellular matrix and cytoskeleton. The proliferative activity of the fibrosis-derived fibroblasts did not significantly differ from that of normal breast fibroblasts. Both cell types required serum to grow and did not form colonies in soft agar. Cells from 2 of the 3 fibroses analyzed displayed aneuploid karyotypes with multiple structural abnormalities. All of the fibroblastic cells produced types I, III and V collagen, fibronectin and vimentin. However, in contrast to normal breast fibroblasts, fibrosis-derived cells produced high amounts of oncofetal fibronectin. In addition, fibrosis fibroblasts also expressed the a-actin isoform which is specific for smooth-muscle cells. These results suggest that post-radiation fibrosis in malignant breast contains atypical fibroblasts with fetal and myofibroblastic characteristics.