Effects of antiserum to S-100 protein (AS-100) in high (HC, corresponding to antiserum dilution 1:5 - 1:50) and low (LC, 1:10(12)) concentrations were studied in identified snail neurons and rat hippocampal slices. HC-AS-100 changed the frequency of action potential generation in spontaneously active neurons and blocked formation of long-term potentiation in mossy fiber synapses. LC-AS-100 alone did not affect any characteristic measured in our experiments, but 20 min pre-incubation of snail ganglia or hippocampal slices with LC-AS-100 abolished the effects of the same antibodies in HC. Simultaneous addition of both LC and HC did not prevent the development of HC-AS-100 effects. It seems possible that LC-AS-100 is capable of interaction with neuronal cells modifying their reaction to HC-AS-100.