The neurooncogenicity recently isolated strains of the human polyoma virus, JC virus, was determined by intracerebral inoculation of newborn Syrian golden hamsters. All three strains produced malignant brain tumors in a majority of inoculated animals during a 6.5-month observation period. The results obtained with the MAD-2 strain, 19 of 20 animals with cerebellar medulloblastomas and 0 of 20 animals with pineal gland tumors, were quite similar to those observed previously with the prototypic strain of JC virus, MAD-1. Inoculation of the MAD-4 strain, however, resulted in 10 of 22 animals with pineal gland tumors and only 10 of 22 animals with tumors in the cerebellum. The MAD-3 strain was neurooncogenic, but too few animals lived to be weaned to provide significant additional information. The basis for the apparent predilection of the MAD-4 strain for the pineal gland is unknown. Two hamsters in the experiment developed extracranial neuroblastomas.