Background: Organophosphorus compounds (OP's) increase endogenous acetylcholine levels by inhibiting acetylcholinesterase. Their suppression of the immune responsesmight be due to direct actionofacetylcholine on the immune system. Objective: To investigate the effect of organophosphorus pesticide, parathion (0.2 mg/kg/day) for 14 consecutive days on the hematological parameters and the immune response of rabbits  after different time intervals (1 hour, 24 hours, 1 week, 2 weeks and 4 weeks after the end of treatment). Methods: Blood samples were analyzed for hemoglobin content (Hb), packed cell volume (PCV), erythrocytes and leukocytes. The cellular immunity was assessed by lymphocyte proliferation response to mitogens; phytohemagglutinin (PHA) and lipopolysaccharide (LPS) and humoral immunity was measured by plaque-forming cell (PFC) generation and hemagglutination titer (HA). Also, nonspecific immunity was assayed by phagocytic activity. Results: They showed that parathion caused a significant increase of total leukocytes and monocytes, while blood erythrocytes, Hb and PCV were insignificantly reduced. Parathion caused a pronounced suppressive effect on the cellular immunity (lymphocyte proliferation response to PHA and LPS) and humoral immunity (PFC and HA). Also, a significant reduction in nonspecific immunity was observed. The suppressive effect of parathion on immune response was time dependent.  Conclusion: The results of the present study suggested that the determinations of hematological and immunological parameters are useful tools for evaluating the toxic effects of parathion on animals.