Objective. To investigate the immunomodulatory activity of a traditional Sri Lankan concoction of Coriandrum sativum L. and Coscinium fenestratum (Gaertn.) Colebr., which is a Sri Lankan traditional medicine used to relieve inflammation and cold. Methods. In vivo anti-inflammatory activity was tested using carrageenan-induced rat paw-edema model. Mechanism of anti-inflammatory activity was assessed by investigating the production of nitric oxide (NO), expression of iNOS enzyme, and reactive oxygen species (ROS) by rat peritoneal cells. The membrane stabilizing activity was also tested. The antibody response was determined by assessing the specific haemagglutination antibodies raised against sheep red blood cells. Results. The three doses of freeze-dried concoction used ((human equivalent dose (HED)—183 mg/kg) 2 × HED and 1/2HED; n = 6 rats/group) showed significant inhibition of paw edema compared to water control at 3rd–5th hours (p<0.05). Both HED and 1/2HED exhibited marked anti-inflammatory activity (72–83% inhibition at 4th-5th hours; p<0.05). The HED of the concoction showed significant inhibition of NO (77.5 ± 0.73%, p<0.001) and ROS production (26.9 ± 2.55%; p<0.01) by rat peritoneal cells. Inhibition of NO production in the concoction treated rat peritoneal cells was confirmed by the lack of iNOS expression. The concoction also exhibited significant membrane stabilizing activity (IC50 = 0.0006 μg/ml; p=0.001). HED resulted in a significantly high induction of specific antibody production against SRBC antigens as detected by SRBC haemagglutination assay (mean day 14 titers 253.3 compared to control: 66.7) (p<0.01). Conclusions. The traditional Sri Lankan concoction of C. sativum and C. fenestratum demonstrated potent in vivo anti-inflammatory activity, significant reduction of ROS, and NO production by rat peritoneal cells and the lack of iNOS expression confirmed the low NO production. The increased membrane stability also supports the anti-inflammatory activity of the concoction. Further, this concoction induced a significantly high antibody response reflecting its immunostimulatory activity. Together these results scientifically validate the therapeutic use of the concoction of C. sativum and C. fenestratum in Sri Lankan traditional medicinal system for immunomodulatory effects.