Repeated intravenous infusions of radioiodinated human albumin and human fibrinogen into normal healthy baboons over a six- to twelve-month period were used for measurements of plasma volume. The radioiodinated fibrinogen gave values for plasma volumes that were quite variable and 20-25% lower than those obtained using radioiodinated albumin. During this period of repeated infusions, there was no apparent reduction in the survival of the human protein in the baboon circulation that would indicate immunization had occurred. Radioiodinated baboon fibrinogen and human fibrinogen administered to the same baboon showed similar survival patterns.