The subclass pattern of red cell bound IgG autoantibody was studied in 304 patients on 426 occasions. Subclass interrelationships with time, other cell bound immunoglobulins (IgM, IgA), amount of bound IgG and serum haptoglobin levels were investigated using population proportions; because of the multiple statistical tests, P<0.01 was required for significance.IgGl was most common, being found in 98% of cases and as the sole subclass in 64%; multiple subclasses occurred in 34.5%. The IgG subclass pattern possibly changed with time (P2 and800 and<400 molecules IgG per red cell respectively). Multiple subclasses (P0.01), but not IgG3, were possibly associated with low haptoglobin levels; significance was reached, however, if the multiple immunoglobulin effect was ignored. IgG subclass interrelationships are clearly complex and require strictly defined populations for their study.