ONE of the earliest papers1 on Rift Valley fever included pigs in the list of refractory species because Daubney, Hudson and Garnham2 had failed to elicit a clinical response in two pigs they had injected. Later, however, Weiss3 directed attention to the circumstantial field evidence that sows had aborted on farms in South Africa where sheep had died. Nevertheless, when Easterday, Murphy and Bennett4 injected three young pigs intraperitoneally with doses of 102.2 and 103.2 mouse LD50 of Rift Valley fever virus there was no clinical response, no viraemia and no development of antibody. Preliminary findings, herein recorded, suggest that the innate resistance of pigs is not absolute, the dose of virus being critical.