Antibody to rat myelin basic protein (MBP), produced in rat or rabbit, and antibodies to bovine serum albumin (BSA) raised in rats, were transferred to normal adult Lewis rats. Recipients were assayed at similar intervals over a period of 4 weeks for serum-borne antibody activity in a radioimmunoassay. It was found (1) that the activity of both rat and rabbit antibodies to MBP showed a more rapid decline in the first 4 hr after injection than did the rat anti-BSA: (2) that both rat and rabbit and anti-MBP activity showed a preferential loss in antibodies of higher affinity; (3) that anti-MBP activity eventually reappeared and actually had more than twice the half-life of that found for anti-BSA or expected for normal immunoglobulin; (4) that antibody activity to MBP did not decay smoothly, but tended rather to oscillate widely between consecutive assays as it declined; and (5) that antibody activities distinguished on the basis of their relative affinities for MBP decayed at slightly different rates, and sometimes fluctuated asynchronously. These observations are consistent with the view that endogenous factors, antigenically resembling small fragments of MBP (MBP-SF) are readily accessible to, and react with, humoral antibody in the normal rat; that the fragments are too small to cause immune clearance of anti-MBP from the circulation; and that fragments representing different regions of the MBP molecule do not necessarily appear at the same time. On the basis of in-vivo studies with radioiodinated anti-MBP antibodies, in which no observable localization of anti-MBP antibodies was obtained, it was concluded that the fluctuations, longer half-lives, and asynchronous affinity variations in activities more likely reflected variable levels of site saturation of circulating antibodies rather than actual temporary clearance of antibody molecules from the circulation.