The target enzyme in organophosphorous-induced delayed neuropathy (OPIDN) has been designated neuropathy target esterase or neurotoxic esterase (NTE). NTE activity can be measured in blood lymphocytes and platelets, which could be of use as biomonitors in man at risk for the development of OPIDN. Separation of lymphocytes and platelets from whole blood, recovery, purity, storage and expression of data were examined. A substantial amount of the NTE activity of a human lymphocyte preparation made using Ficoll/Pacque was due to contamination by platelets; further purification was achieved by sucrose-gradient centrifugation. In an easily prepared sample of human platelets less than 10% of NTE was associated with contaminating white cells. We were unable to preserve NTE activity of platelets or lymphocytes at -80 "C either 'dry' or with added buffer and glycerol. In 68 male subjects, NTE activity in platelets averaged 8.36 L 1.54 nmol min-' mg protein-' and NTE activity in lymphocytes, obtained from blood after removal of platelets, 13.34 2 2.42 nmol min-' mg protein-'. A good correlation was found between platelet and lymphocyte NTE activity. NTE activity in platelets may be a preferable method for measuring exposure to axonopathic organophosphorous compounds because of the ease and purity of separation. No correlation with other neuropathic risk factors such as age, smoking and alcohol intake was noted.