A novel thiazolidine-2-thione functionalized chain transfer agent (CTA) was synthesized and used as a reversible addition-fragmentation chain transfer (RAFT) polymerization agent to prepare well-defined poly-N-(2-hydroxypropyl) methacrylamide (PHPMA). The polymer chains had pre-designed molecular weights, narrow polydispersities and were chain-end functionalized. On incubation with protein (lysozyme) under different pH conditions, PHPMA was conjugated to the protein surface via covalent amide bonding. The bioactivity of the lysozyme-PHPMA conjugates was assessed using Micrococcus lysodeikticus (Ml) cells as substrates. The number of polymer chains attached to the protein could be controlled by both the pH of the conjugation reaction and the molecular weights of the polymers, thereby influencing significantly the bioactivity of the protein-polymer conjugates.