The grafting polymerization of methyl methacrylate (MMA) and cellulase from henequen (Agave fourcroydes) is investigated as a function of the initiator concentration( cerium-and-ammonium nitrate) and the monomer/cellulose ratio. The formation of cellulose-g-PMMA is confirmed by IR spectroscopy, DSC, and TGA. Both the initiator concentration and the MMA/cellulose ratio have a strong influence over the grafting parameters and over the molecular weight of the grafted PMMA. A higher initiator concentration and a lower monomer/cellulose ratio result in a lower molecular weight of the grafted polymer. Increasing the amount and the molecular weight of the grafted PMMA increases the compatibility of the fibers with SAN and PVC, as demonstrated by a mechanical test and scanning electron microscopy. SAN and PVC composites made with grafted cellulose exhibit higher flexural and tensile moduli, respectively, than those produced with the ungrafted fibers. Both moduli increase as the amount of reinforcement increases.