The surfaces of normal hamster embryo fibroblast (HEF) cells were examined by scanning electron microscopy. Surface characteristics of HEF cells were compared to those of cells derived from a primary tumor induced in hamsters following s.c. inoculation of herpes simplex virus type 1-transformed HEF cells (14-012-8-1) and to the surfaces of cells derived from a metastatic tumor to the lung induced by the same cells. The most obvious difference in the surface characteristics of the examined cells was the morphology of the microvilli. In the few HEF cells that possessed microvilli, the distribution was uneven, and the lengths of the microvilli and the filopodia were variable. However, the surfaces of both tumor cell lines showed large numbers of microvilli which were evenly distributed over the surface of the cells, giving an almost "hairy" appearance. Long filopodia were occasionally observed on the surface of the primary tumor cell line and on the cell line derived from the metastatic tumor. Ruffles and blebs were occasionally observed on HEF cells and on the primary tumor cells but were not seen on the cells of the metastatic tumor.