We developed a new method that yields biologically potent therapeutic proteins such as interferon a2b (IFNa2) and human growth hormone (hGH) in high yields and with increased serum half-lives when expressed as arabinogalactan-protein (AGP) chimeras in cultured tobacco cells. The chimeric glycoproteins typically gave 500–1800 fold greater secreted yields than their non-glycosylated controls. Importantly, the chimeric glycoproteins showed an increased in vivo serum half-life of up to 13 fold while their biological activities remain similar to native IFNα2 and hGH.