The expression of smooth muscle (SM) and non-muscle (NM) myosin heavy chain (MyHC) isoforms has been studied in fibroblastic cells of different fibrotic lesions (hypertrophic scars, Dupuytren's nodules and stromal reaction to mammary carcinoma) and in epithelial cells of non-neoplastic and neoplastic mammary glands, using anti-myosin antibodies in immunofluorescence and Western blotting. Two antibodies were specific for SM-MyHC isoforms (SM1 and SM2) and three antibodies were directed against different sequences of NM-MyHC isoforms. Myofibroblasts containing SM-MyHC were present in a variable number of cases of the different lesions: 1 of 11 hypertrophic scars, 3 of 9 Dupuytren's nodules and 20 of 25 breast cancers. The distribution of NM-MyHC sequences recognized by our antibodies was heterogeneous in fibroblasts from normal dermis and mammary stroma, but became homogeneous in myofibroblasts from all the pathological conditions examined. Moreover, the expression of these MyHC sequences differed in normal mammary epithelium when compared with invasive carcinoma. These results show that cellular modulation from fibroblast to myofibroblast may be accompanied by the appearance of SM-MyHC and is characterized by a uniform expression of MyHC of NM type, and that tumour progression in mammary epithelial cells may be paralleled by the disappearance of a specific NM-MyHC sequence. This suggests that MyHC modulation participates in the process of fibrosis as well as in the process of malignant epithelial transformation.