Investigations have been undertaken to determine the effect of occupational exposure to organophosphoric pesticides on the behaviour of NBT-dye reduction and spontaneous E rosette formation tests. A significant decrease of the values of the examined cellular parameters of human immunity and of the absolute lymphocyte count in peripheral blood was found in 31 workers, as compared to 30 controls. Moreover, the NBT-dye reduction decrease showed a positive linear correlation with a reduced acetylcholinesterase (AChE) activity. The exposure time showed a negative linear correlation of the phagocytic index of the NBT-dye reduction test. It is concluded that depressed NBT-dye reduction and diminished spontaneous E rosette formation may be regarded as a probable mode of the effect of organophosphoric chemicals on metabolic and membrane damage of human cells. A possible role of endogenous acetylcholine poisoning in the development of these pathologic conditions is also discussed.