Summary Plants have recently received a great deal of attention as a means of producing recombinant proteins. Despite this, a limited number of recombinant proteins are currently on the market and, if plants are to be more widely used, a cost‐effective and efficient purification method is urgently needed. Although affinity tags are convenient tools for protein purification, the presence of a tag on the recombinant protein is undesirable for many applications. A cost‐effective method of purification using an affinity tag and the removal of the tag after purification has been developed. The family 3 cellulose‐binding domain (CBM3), which binds to microcrystalline cellulose, served as the affinity tag and the small ubiquitin‐related modifier (SUMO) and SUMO‐specific protease were used to remove it. This method, together with size‐exclusion chromatography, enabled purification of human interleukin‐6 (hIL6) with a yield of 18.49 mg/kg fresh weight from leaf extracts of Nicotiana benthamiana following Agrobacterium‐mediated transient expression. Plant‐produced hIL6 (P‐hIL6) contained less than 0.2 EU/μg (0.02 ng/mL) endotoxin. P‐hIL6 activated the Janus kinase‐signal transducer and activator of transcriptional pathways in human LNCaP cells, and induced expression of IL‐21 in activated mouse CD4+ T cells. This approach is thus a powerful method for producing recombinant proteins in plants.