The immunohistochemical features of tumors from 7 Holstein cows showing fibroblastic, myofibroblastic, smooth muscle or pericytic differentiation are described. In cases 1 and 2, the tumors were characterized by neoplastic fibroblasts immunostained with vimentin alone, but some cells appeared to transiently express alpha smooth muscle actin (SMA) during tumor development. Cutaneous (case 3) and nasal (case 4) myofibroblastomas, which were characterized by SMA and fibronectin positivity, with a minority of cells showing positive reactivity for desmin, were readily distinguishable from tumors of fibroblasts, smooth muscle cells and pericytes by fibronectin staining. The same staining pattern was seen in a vulvar myofibroblastoma (case 5), which differed from a vaginal leiomyoma (case 6) coexpressing SMA and desmin in nearly all neoplastic cells. Both neoplasms, however, had estrogen receptor (ER), and may be derived from immature stromal cells distributed in the vulvovaginal region. A hemangiopericytoma (case 7) resembled myofibroblastomas in cytoskeletal immunophenotypes, but lacked fibronectin. Benign or locally invasive soft tissue tumors with spindle cell morphology and expression of SMA could be divided into several types by immunostaining for SMA, desmin, fibronectin and ER.