Smithburn (1949) reported that humoral immunity to Rift Valley fever (R.V.F.), transmitted from an immune ewe to her twin lambs, persisted for about five months and was not affected by weaning. He postulated that this might be of practical value in protecting young lambs during the period of maximum susceptibility to Rift Valley Fever virus. In the Republic of South Africa, arthropod-borne diseases of sheep such as Rift Valley fever, Wesselsbron disease and bluetongue occur seasonally during the late summer and autumn, and almost invariably disappear soon after the first frosts of winter. Bearing this in mind, practical use is made of transmitted passive immunity to protect the lambs. Autumn lambing is advocated, and farmers are advised to immunise all their sheep during the spring, first with a polyvalent bluetongue vaccine and later with a combined Rift Valley fever-Wesselsbron vaccine. On account of the abortifacient and embryotropic properties of the neuroadapted Rift Valley fever and Wesselsbron disease viruses which form the basis of the live attenuated virus vaccine, prophylactic immunisation is restricted to non-pregnant animals. At present, for the preparation of the Rift Valley fever vaccine, Smithburn's neurotropic attenuated strain of virus at the 102nd mouse intracerebral passage level is used. Mulligan (1937) prepared a similar type of vaccine from a neurotropic strain of virus at the 92nd mouse passage level. He reported that “the vaccine proved safe for lambs six weeks old and produced a good immunity. Tested in the field, on a large number of pregnant ewes and newly born lambs, this vaccine was found to be unsafe for the newly born lambs and to cause abortion in some of the pregnant ewes. Vaccination of pregnant ewes, moreover, did not appear to result in the transfer of the immune bodies through the colostrum to the lamb although, in this connection, it must be stated that lambing had started at the time vaccination was practised and the interval between inoculation and the birth of a lamb was never more than a few days. It became apparent, however, that the vaccine, while it might be of service in protecting maiden ewes, was not suitable for controlling an outbreak in pregnant ewes and very young lambs." Kaschula (1953) using Smithburn's neurotropic virus at the 86th passage level reported abortions among pregnant ewes and the death of lambs shortly after birth under field conditions in South Africa. The investigations reported in this communication were undertaken initially to determine the effect of the strain of Rift Valley fever virus incorporated in the vaccine on pregnant ewes and day-old lambs. During the course of the investigations valuable information was obtained on the nature and duration of transmitted immunity in lambs and the immune response of day-old lambs to the neurotropic strain of Rift Valley fever virus.