Currently available specific synaptic markers have made it possible to estimate the synaptic density by immunochemical techniques. In the present study we labeled the neocortical presynaptic terminals in histological sections of human autopsy tissue with a monoclonal antibody against synaptophysin. The characteristic granular neuropil reaction was quantified by measuring the average optical density (OD) in the different layers of the parietal cortex with the aid of image analysis equipment. The raw neuropil OD was corrected by subtracting the OD of the white matter in the same section. Our study showed that consistent microdensitometric results can be obtained on 5-microns paraffin sections from specimens with less than 8 hr of post-mortem time before fixation, incubated with 5 micrograms/ml of anti-synaptophysin. The corrected OD measurements were slightly larger in neocortical layers II, III, and V than in layers I, IV, and VI, but the differences were not statistically significant. In area 17, layer IV was denser than the others. We conclude that with certain precautions this method can be used to measure relative amounts of synaptophysin-like immunoreactivity and to infer the density of presynaptic boutons in human situations and in animal models.