Mouse neuroblastoma cells containing intracisternal type A particles were treated with iododeoxyuridine and dexamethasone to induce the release of type C oncornavirus particles. For 5 days after treatment, antigenic markers and DNA polymerase activities specific to particles of each of the two types were assayed in the cells and in pellets obtained by high-speed centrifugation of the culture fluid. There was a marked release of C-particle antigen (p30) and DNA polymerase activity in extracellular particulate form, reaching a maximum on day 3 after treatment and falling thereafter. In contrast, no extracellular A-particle antigen was detected, and A-particle-specific DNA polymerase activity in the medium pellets did not increase from the original very low level. Electron microscopy confirmed the presence of free type C virus particles, but not intracisternal type A particles, in the culture fluid. Although intracellular levels of C-particle antigen rose 20- to 30-fold per milligram of cell protein, intracellular A-particle antigen and DNA polymerase activity did not vary more than two-fold. The relative rate of A-particle synthesis in the treated cells, as judged by incorporation of radioactive amino acids into the major structural protein (P73), was also unchanged over the period of observation. Thus, the induction of type C virus particle formation in cultured neuroblastoma cells had no detectable effect on the quantity, synthesis rate, or location of intracisternal type A particles.