We evaluated 10 microfilters for their ability to filter diluted sera without removing immunoglobulins (Ig) G, A, and M, albumin, and transferrin. In general, filters containing cellulose nitrate remove IgG from solution, the amount adsorbed being proportional to the IgG concentration in the solution. With some sera we noted IgA and IgM adsorption to cellulose-nitrate-containing filters, but there was no significant adsorption of albumin or transferrin to any of the filters. We also found that cellulose-nitrate filters adsorbed IgG from antiserum, with consequent loss of titre as seen in a nephelometric assay. Adsorption of IgG was not seen if the filters were prewashed with polyethylene glycol or if the antisera contained polyethylene glycol. With a sufficiently large antibody excess in the nephelometric assay, this loss of titre through filtration becomes undetectable.