RNA 4, the subgenomic coat protein messenger of alfalfa mosaic virus, was loaded with small amounts of coat protein in a reaction in which complete virions were the protein donor. In such a reaction the protein subunits attach to the high-affinity binding sites near the 3' end of RNA 4 [Houwing, C. J., & Jaspars, E. M. J. (1978) Biochemistry 17, 2927-2933]. At a ratio of up to 13 coat protein subunits to 1 mol of RNA 4, complexes with one and three protein dimers, designated complex I and complex III, respectively, were formed. These complexes were isolated by preparative electrophoresis in 4% polyacrylamide gel. At a large excess of the protein donor (280 protein subunit/mol of complex), both complexes I and III were converted into uniform complexes with 10 protein dimers. There were no indications for stable intermediate complexes. A model is suggested for the structure of the complexes which is based on the model proposed for the protein coat of alfalfa mosaic virus [Mellema, J. E., & Van Den Berg, H. J. N. (1974) J. Supramol. Struct. 2, 17-31]. The complexes possibly serve as successive stages in virion assembly. More intriguingly, the complexes could be of regulatory significance. Since the four RNA species of alfalfa mosaic virus have an extensive 3'-terminal homology, and since 3'-terminal interaction with coat protein subunits is thought to be a process leading to recognition of the viral genome by the viral replicase and thus to infectivity, complexes analogous to complexes I and III could represent the infectious forms of the genome RNAs.