The rate of neutralisation of herpes simplex virus 1 was increased by up to more than five hundred-fold when the virus suspension and antiserum were each diluted to one hundred-fold in water instead of phosphate buffered saline. This phenomenon, which was observed for two human positive sera and a rabbit purified polyclonal antibody, may represent an unrecognised homeostatic mechanism where neutralising antibody is 'dilution-fast' under physiological conditions of transudation or pathological conditions of inflammation.