Trifoliate orange seed extracts (TSEs) were made using either distilled water (TW), ethanol (TE), or n-hexane (TH), to measure total polyphenol contents, DPPH and ABTS radical scavenging activities, and anti-complementary activity. The results showed that the total polyphenol content showed higher value at TE (235.24 ${mu}g/mL$, p<0.05) than those of TW (132.65 ${mu}g/mL$) and TH (165.44 ${mu}g/mL$) at 10 mg/mL and TE exerted the highest DPPH radical scavenging activity (61.77%, p<0.05), which occurred in the following order: TE TW (56.87%)>TH (39.78%). The results of ABTS radical scavenging activity showed that TW (34.26%) and TE (31.81%) showed similar activities, which were higher than TH (12.74%, p<0.05). Anti-complementary activity of TE (61% at 500 ${mu}g/mL$) showed a higher activity when compared with the positive control (60% at 1,000 ${mu}g/mL$) polysaccharide-K (PSK), a known immuno-active polysaccharide from Coriolus versicolor. Consequently, among TSEs, TE is a byproduct from trifoliate orange and could be an important source of dietary polyphenolic antioxidant compounds and immunopotentiating activity, including complement activation.