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 less than 0.01 was required for significance. IgG1 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 (P less than 0.02, greater than 0.01), populations being compared at 6 and 12 months. There was a highly significant and important correlation between multiple IgG subclasses and multiple immunoglobulin coating; in our further studies, this necessitated the use of samples where only cell bound IgG was increased. Multiple IgG subclasses strongly correlated with larger amounts of cell bound IgG, groups with greater than 2 and less than 1 OD units by the enzyme-linked direct antiglobulin test being compared (approximately greater than 800 and less than 400 molecules IgG per red cell respectively). Multiple subclasses (P less than 0.05, greater than 0.01), 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.