In vitro cultivation of medulloblastoma cells was successfully established from hamster brain tumors induced by simian adenovirus SA7. These tumor cells possess morphological features in cultures that are very similar to those of the original tumor. Both tubular and rosette growth patterns were evident. At the ultrastructural level only small numbers of cytoplasmic organelles could be detected in the tumor cells, typical of generally immature and/or undifferentiated cells. The proliferation of these tumor cells depended upon properly dense initial platings. In addition, it was found that single medulloblastoma cells could be stimulated to produce colonies if treated for 10 days with conditioned medium. Inoculation of these cells into synergenic animals resulted in 100% uptake. Survival of the hamsters was directly correlated with the number of cells injected. This new medulloblastoma cell line may prove to be a useful model for experimental brain tumor studies.