The failure to detect yellow fever-virus antibody in baboons known to have been previously vaccinated suggested an attempt to explain this finding. Accordingly, a study was initiated to test baboon and human serum specimens before and after vaccination for the presence of yellow fever neutralizing antibody, two different preparations of vaccine for determination of antibody being employed. Neutralization tests in mice were employed with 1:5 serum dilutions. The results indicated that good immunologic responses are obtained if the vaccine is handled and used according to specifications. Failures in vaccination arise in the field presumably as a result of mishandling of this highly labile material.