Scanning electron microscopy (SEM) was used to study the surface configuration of SV40 virus-transformed 1 day old Syrian hamster brain cells and transformed cell-produced tumors and the alterations in cell membrane configuration that become evident with the development of invasiveness. Cell cultures of dissociated cerebral cortex, brain-stem, cerebellar vermis and cerebellar hemisphere were transformed to a neoplastic state with SV40 virus. The transformed cells were then inoculated intracerebrally into hamsters to produce intracranial tumors, and cell cultures were obtained from these tumors. Invasiveness was determined by reinoculating the transformed or tumor cells intracerebrally into hamsters and examining histological sections of the resulting tumors. Cultures of the transformed and tumor cells were also critical point dried and studied with SEM. Tumors of cerebral cortex and brainstem derivation (Group I) had the histological appearance of astrocytomas and were invasive for adjacent normal brain. With SEM, they were flat, epithelioid, and surrounded by ruffles and microvilli. They grew only in monolayers and showed few of the rounded forms with blebs and/or microvilli typical of mitosis. Tumors of cerebellar vermis and cerebellar hemisphere derivation (Group II) had the histological appearance of primitive neuroectodermal tumors and were not invasive. With SEM, they were more rounded, fusiform or stellate, and lacked surface specializations at their borders. They exhibited multilayered growth and the rounded forms associated with mitosis. Tumor cells of both groups had more surface activity than transformed cells. These observations give some support to the concept that a decrease in contact inhibition of locomotion and an increase in density-dependent inhibition of locomotion and in mobilization are essential for the development of invasiveness in tumors.