U. S. Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health, National Institute of Allergy and Infectious Diseases, Rocky Mountain Laboratory, Hamilton, Montana 59840
Neutralizing antibodies formed after inoculation of man with a purified Colorado tick fever vaccine have persisted in most instances for 5 years or more. Intradermal inoculation of small amounts of vaccine for skin tests, even that long after original vaccination, resulted in a significant rise in titer of neutralizing antibody. There was no evidence of development of hypersensitivity to any of the components in the vaccine. Results of prevaccination skin tests did not differ significantly from those obtained in tests given after several inoculations of vaccine.