Recent epidemics of yellow fever in eastern Africa have stimulated serological surveys in the Karamoja district of Uganda, the northern frontier district of Kenya and the Giohar district of Somalia. All sera collected in the surveys were screened for group B arbovirus antibody using the HI test. Yellow fever immunity was confirmed by the mouse-protection test.No yellow fever immunity was found in sera collected from a residential tribal group at Karamoja but a small number of samples from persons who had previously lived outside the district showed immunity. Immunity was detected in sera from areas of the Northern Frontier district of Kenya. It is thought that this immunity may, in some areas, have resulted from extensions of the Ethiopian epidemic of 1960-62 into the region, but in another area of the district the immunity seems to have arisen from continuing focal transmission within Kenya. Yellow fever immunity was also detected in Somalia.