Human sera collected during a suspected yellow fever outbreak in Mabudi, Benue-Plateau State of Nigeria were examined for virus content by mouse inoculation, and presence of antibody by complement fixation (CF) and neutralization (N) tests in baby mice. No virus was isolated from 116 sera examined. 40 and 41 of the 358 sera tested were positive for yellow fever and UgMP 359 CF antibodies respectively, with 12 possession CF antibodies to both viruses. In neutralization tests, 91 and 99 of the 243 sera tested were positive for yellow fever and UgMP 359 N antibodies respectively with 37 positive for both viruses. Tissues from one hundred and twelve animals; 77 rodents and 45 birds were also tested for virus content and found negative. None of the 9 rodent and 3 bird sera was positive for N antibody to either virus.