IgG subclass concentrations have been measured in the sera of 172 healthy adults using monoclonal antisera.'Normal ranges' are given. The findings were similar to those of previous studies of European populations but differed from the findings of the only other study of a large British population. In particular, low proportions of IgG4 in a British population have not been confirmed. IgG4 concentrations were higher in males than females (P= less than 0.00003) and IgG3 concentrations were higher in females than males (P= 0.034).