Eleutherococcus senticosus somatic embryos were cultured in Murashiage and Skoog medium in balloon type bubble bioreactor (working volume 2-1) to investigate the effect of light on biomass, secondary metabolites, stress levels (lipid peroxidation and hydrogen peroxide (H2O2) and changes of antioxidant enzymes. Among the different light sources, biomass accumulation was found optimum under fluorescent (171) and mixed blue plus far-red (B1 + Fr) irradiation. Accumulation of eleutheroside E (51%) and El (21%) was highest at red light while fluorescent light produced highest amount of total phenolic (2.7%), total flavonoid (34%) and chlorolgenic acid (14%) contents compared to dark (control) grown mature embryos. Higher H2O2 content, malondialdehyde (MDA) content and lipoxygenase (LOX) activities was observed at red light treated embryos compared to dark (control) grown embryos. Antioxidant enzymes, monodehydroascorbate reductase (MDHAR), catalase (CAT), glutathione S transferase (GST) and superoxide dismutase (SOD) which are playing important role for the detoxification of harmful substances were also induced in red light irradiated embryos. However, ascorbate peroxidase (APX) took a little part in detoxification of H2O2 due to its sensitivity to red light. Therefore, reduced APX activity in red light treated embryos was observed. While APX activity was maximum in fluorescent light treated embryos compared to other light sources. This reflects the sensitivity of the enzyme to the different light. The study concludes that embryo could grow under different light irradiation and protect themselves from toxicity of light sources by altering various phenolic compounds and antioxidant enzymes. (c) 2005 Elsevier Ltd. All rights reserved.