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Two spraymen working in public health occupations in Alexandria, Egypt, experienced acute toxicity resulting from exposure to diazinon. Symptomatology was similar to that previously reported for exposure to parathion or other organophosphorus insecticides. Plasma and red blood cell cholinesterase activity values were determined in blood samples obtained from both individuals at various times after the incident. Cholinesterase activity showed a marked reduction up to 18 days after exposure. Blood cholinesterase activity recovered to approximately 90% of the normal level of activity 28 days after the poisoning incident in one individual. This activity recovered to about the same level in the other individual, but after only 20 days from the poisoning date. Experimental results suggested that this acute toxicity resulted from unsuitable storage conditions of the emulsifiable concentrate formulation of diazinon. The diazinon that was applied was stored in "tin" containers made of tin-plated sheet steel. The emulsifiable concentrate (60%) was not in compliance with the World Health Organization's standard specifications regarding the emulsion stability tests because of the presence of crystals in the emulsifiable concentrate. A sample of this crystalline material was analyzed. Gas chromatographic analysis combined with mass spectrometric techniques failed to identify intact diazinon in samples of that material. The sample represented virtually complete conversion of diazinion into transformation products. Sulfotepp and monothiono-TEPP were two of the identified products in the sample, both of which are much more toxic than diazinon.