The recessive mutation lethal (2) giant larva4 (1(2) gl4 ) of Drosophila melanogaster causes the development of malignant tumors in the whole brain of homozygous larvae. A mutant brain fragment implanted into the abdomen of a wild-type adult female kills the host in about 10 days. Neuroblasts in situ in 1(2) gl4 larvae showed normal karyotypes, but, when cultured in adult abdomens for one transfer generation, about 10% of the cells showed chromosome aberrations. Subculturing the neuroblasts for four transfer generations showed that malignancy (i.e., lethality to the host) as well as chromosomal abnormalities increased with time of subculture. Many virus-like particles were detected in 1(2) gl4 neuroblasts after in vivo culture, whereas no such particles were detected in 1(2) gl4 neuroblasts in situ in larvae. These particles contained RNAs homologous in sequence to the DNA of the movable element copia. They were indistinguishable from previously identified retrovirus-like particles in cultured Drosophila cells. It is proposed that the 1(2) gl4 mutation reduces the genome integrity, resulting in transplantation-triggered genetic abnormalities, such as chromosomal abnormalities, increased transcription or replication of copia elements, and production of retrovirus-like particles.