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BACKGROUND Spirulina platensis is rich in β-carotene, which possesses many important biological activities. This study investigated the ultrasound-assisted extraction and purification of β-carotene from Spirulina platensis by using response surface methodology ( RSM), determined its antioxidant capacity in vitro and explored its hypoglycaemic effect in diabetic mice. RESULTS The raw β-carotene extract with a concentration of 1942.14 ± 10.03 µg mL−1 was obtained at the optimised condition by RSM (0.40 of the solid-liquid ratio, 51% of the extraction power, and 17 min of the extraction time), and the purity of evaporated β-carotene extract reached 816.32 ± 10.57 mg g−1 after purified by a NKA-9 resin with a sampling and elution rate of 1 mL min−1. The β-carotene extract scavenged 1,1-diphenyl-2-picrylhydrazyl and hydroxyl free radicals with the highest ratios of 44 ± 0.26% and 35 ± 0.45% respectively, and exhibited strong inhibiting capacity on anti-lipid peroxidation. The blood glucose level of streptozotocin-induced diabetic mice was significantly reduced from 15.81 ± 1.71 mmol L−1 to 8.10 ± 0.88 mmol L−1 after 10 d administration of the β-carotene extract [100 mg kg−1 body weight ( BW)], and the increased food and water intakes in the diabetic mice were also significantly relieved after β-carotene treatment. CONCLUSION Our results suggested that extraction of β-carotene from Spirulina platensis had potential prospects in scaled-up industrialisation and healthcare applications. © 2015 Society of Chemical Industry [ABSTRACT FROM AUTHOR]