The immunotoxic effects of the organophosphorus plant growth regulator ethephon and the probable protective effects of green tea were investigated in mice. Animals received ethephon in the diet at a dose level equivalent to 1/10th oral LD50 for 8 weeks. Red blood cell count, hemoglobin level and hematocrit value were reduced by ethephon treatment, while leucocytosis, neutrophilia, monocytosis and lymphocytopenia were recorded. Significant elevations were observed in activities of serum transaminases and levels of total bilirubin, urea and creatinine. Serum total protein, albumin and albumin/globulin ratio were reduced. Serum hemolyzing antibody titer against sheep red blood cells (SRBC) and levels of serum immunoglobulins were decreased indicating inhibition of humoral immunity. The delayed-type hypersensitivity response to SRBC and the phagocytic activity of polymorphonuclear cells towards candida albicans were suppressed. Levels of malondialdehyde in spleen and thymus were elevated while glutathione levels were reduced. The activities of antioxidant enzymes catalase and glutathione peroxidase were inhibited in both spleen and thymus. Histopathological alterations were recorded in liver, kidney and spleen. Co-administered green tea extract with ethephon ameliorated most of the toxic effects of ethephon.