We expressed HRP on the surface of S. cerevisiae and grew the yeast on synthetic galactosecontaining medium (SG). Flow cytometry revealed that a minority (10%) of the cells successfully expressed the enzyme (SI Figure ). The standard TMB assay showed clearly that HRP-expressing cultures had much higher enzymatic activity than the control, suggesting that the enzyme could be investigated as catalyst for biocatalytic controlled radical polymerization (bioCRP) (SI Figure ). We tested both enzymatic RAFT (bioRAFT) and enzymatic ATRP (bioATRP) for enzyme-mediated CRP. We used N-hydroxysuccinimide (NHS) chemistry to tether either the RAFT chain transfer agent 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPPA) or the ATRP initiator bromoisobutyrate (BiB) to proteins exposed on the surface of the yeast cells. The fluorescamine assay showed that nearly 90% of CTA or the initiator (SI Figure ) reacted with lysines and thus decreased the amount available to fluorescamine. Using carboxyfluorescein-NHS (CF-NHS) as a fluorescent model molecule with similar reactivity, we observed no visible uptake into the cells, suggesting that both CPPA and BiB would be presented on the outer cell surface, together with the surface-displayed HRP (SI Figure ).