An attenuated strain L11A of tobacco mosaic virus (TMV) multiplied like wild type strain L at an early stage of infection in tomato leaves. Four days after inoculation, however, multiplication of L11A was drastically reduced (autoregulation) compared with the constant multiplication of L. In mixed infections, L11A strongly inhibited the multiplication of homologous strain L. Experiments with cucumber mosaic virus (CMV) or tobacco plants revealed that the inhibitory mechanism of L11A is not host-specific but virus-specific, and the autoregulatory mechanism is effective only for TMV. RNA synthesis in L11A infected leaves 4 days after inoculation was studied by polyacrylamide gel electrophoresis. Synthesis of TMV-RNA and its replicative intermediate were strongly inhibited, whereas the replicative form of TMV-RNA and ribosomal RNA were synthesized as in the case of L infection. Synthesis of non-coat-protein was studied by the incorporation of radioactive histidine into subcellular fractions derived from leaves infected with L or L11A for 4 days. Different patterns of the two strains in protein synthesis were noted. At least three proteins were predominantly synthesized in L11A infection. One of them was observed in the mitochondria fraction. From its position in polyacrylamide gel, it could be viral coded 165K protein which is considered to be involved in viral RNA replication. These results suggest that the unique nature of attenuated virus L11A, i.e. autoregulation, resulted from the inhibitory mechanism of viral RNA synthesis due to overproduction of 165K protein and is quite distinct from interferon, intrinsic interference or interference by defective virus.