Abstract: Insolubilized serum protein adsorbent, for example bovine serum albumin, of small particle size is an excellent adsorbent for adsorbable components in a biological fluid such as serum. Macroaggregated and polymerized albumin are particularly suited as adsorbents for unbound thyroid hormone in serum and can be used in diagnostic kits for determining total T.sub.4 and for T.sub.3 uptake.