Infection of dissociated neuron cultures of mice with VSV and its defective particle DI-T was studied using fluorescent light microscopy as well as transmission and scanning electron microscopy. When cultures are infected with wild virus, VSV replicates selectively in neurons, producing cell death within 24-48 hr. Sensory and immature neurons express viral antigen most rapidly. Viral antigen and viral budding sites are detected along the neuron soma and dendrites. When large amounts of DI-T particles are added to the wild virus inoculum, viral growth is completely suppressed in mature neurons, the cell killing effects of VSV are considerably delayed and co-infected cultures survive for 5-16 days. Viral antigen accumulates in cytoplasmic inclusions and on the membrane of neuron cell somas and dendrites in the virtual absence of viral assembly. Identical modulation of VSV infection in mature neuron cultures is obtained when DI-T particles are added before or after the wild virus, but ultraviolet inactivation of DIs completely abolishes their protective effect. Immature neurons or Vero cells cannot be protected from acute cytopathic changes by an equivalent amount of DI particles. Thus DIs interfere with replication and assembly of the wild virus and attenuate cell killing effects in mature neurons in vitro.