The extent of basement membrane confinement among 5 cases of ameloblastoma and a malignant counterpart were studied by immunohistochemistry. For these studies, antibodies to 2 basement membrane components, Type IV collagen and laminin, were studied by immunohistochemistry. Our results show that the tumor islands of ameloblastomas are circumferentially delineated by a linear staining to both antibasement membrane antibodies, and that these findings were consistent for all of the patterns of ameloblastoma investigated. These data suggest that ameloblastomas spread into tissue spaces by expanding their compartmental volumes rather than by secreting proteinases that disrupt their basement membranes compartmental barriers. In contrast, the single case of malignant ameloblastoma investigated was not delineated by circumferential linear basement membrane components. However, this tumor and all of its metastases did contain numerous focal areas of Type IV collagen and laminin immunoreactive materials. Collectively, these studies indicate that the use of specific antibodies to basement membrane components may help to differentiate ameloblastomas from malignant lesions.