The expression of phosphorylated neurofilaments (PNF) epitopes in hippocampal neurons was examined in 5 cases of Alzheimer's disease and 12 brains of schizophrenic patients. A control group (n = 17) consisted of 5 brains of patients with other acute or chronic neurologic disorders and 12 brains of persons who were free of any known neurologic disease. Immunocytochemistry with monoclonal antibodies to PNF was done on formalin-fixed, paraffin sections of hippocampus. Adjacent sections were impregnated with silver. Immunoreactivity to PNF was observed in tangles and plaques of the Alzheimer's brains and also in hippocampal pyramidal cells in 6 of the 12 schizophrenic brains and in 9 of the 17 control brains (2 with neurological disorders and 7 with non-neurological diseases). The positive PNF-stained cell bodies, in silver impregnated sections, generally did not show evident neurofibrillary abnormalities. Enzymatic dephosphorylation prior to immunostaining abolished PNF reactivity. These results suggest the widespread occurrence and relative non-specificity of perikaryonal PNF immunoreactivity in human post mortem brain tissue. We could not exclude the possibility that the hippocampal and brainstem neurons are predilection sites for normal perikaryonal phosphorylation in human brain.