The relative amount of human IgG subclasses and/or subclass-specific determinants were measured in seven commercial i.v. immunoglobulin preparations and one human serum preparation - Paul Ehrlich Institute Plasma Protein Reference No. 2. Quantification was done by means of subclass-specific antisera and a human IgG subclass standard HOO-02 using the recently established rate nephelometric method. The differences between the one human serum preparation and those of normal human serum are much smaller than those between the i.v. immunoglobulin preparations, being for IgG2 the greatest and for IgG3 the smallest. This does not come as a surprise because the various treatment processes used in the manufacture of i.v. immunoglobulin preparations are likely to result in loss and/or masking of the subclass-specific determinants. Although little is known about the genetical ramifications which could arise through the use of i.v. immunoglobulin preparations in which there is variation in the subclass distribution,attention should be given to this subject as passive immunotherapy becomes more popular.