SUMMARY

The best conditions found for infection of tobacco mesophyll protoplasts with alfalfa mosaic virus (AMV) were pH 5"2, 2"5 #g/m1 virus and I/,g/ml poly-L-ornithine. Newly synthesized virus could be detected I5 h after inoculation and the virus concentration reached an estimated IO ° to Io 7 particles per infected protoplast 2 days after inoculation. The study of the replication f plant viruses has been retarded because of the lack of a system in which synchronous multiplication ofvirus particles could take place. Protoplasts isolated from tobacco mesophyll cells can be infected with several plant viruses including some with spherical particles: cowpea chlorotic mottle virus (CCMV; Motoyoshi et al. 1973); cucumber mosaic virus (Otsuki & Takebe, I973); pea enation mosaic virus (PEMV; Motoyoshi & Hull, 1974); a variant of brome mosaic virus (BMV; Motoyoshi, Bancroft & Watts, I974), and others with rod-shaped particles: tobacco mosaic virus (TMV; Takeb