The possible activation of Natural Killing (NK) activity by interferon (IFN) was studied in peripheral blood leukocytes of newborn piglets. It is reported that, although NK is very low during the first week of life, in vitro pretreatment of newborn leukocytes with semi-purified human alpha IFN induces a significant NK enhancement. These data suggest that piglets have pre-NK cells which become fully cytotoxic when exposed to IFN.