Adherence, metabolic burst and chemotaxis of polymorphonuclear neutrophils (PMNs) were examined in 15 children before and seven days after measles-mumps-rubella vaccine administration. In all children, PMN functions were significantly reduced on the seventh day. Adherence, metabolic burst and chemotaxis tested in three subjects one month after vaccination had returned to normal values. Only two children presented transient hyperpyrexia. We conclude that measles-mumps-rubella vaccine administration suppresses PMN functions without clinical consequences. This is probably because attenuated strains of vaccine viruses do not replicate in lymphoid tissues as extensively as do wild-type strains.