Platelet membrane IgG levels were measured by the complement lysis inhibition test, and were correlated with serum IgG levels in 29 patients with elevated immunoglobulins due to multiple myeloma, benign monoclonal gammopathy, systemic lupus erythematosis, rheumatoid arthritis, benign hypergammaglobulinaemic purpura and systemic infection. Platelet membrane IgG was increased in proportion to the elevation of serum IgG with a highly significant correlation coefficient (0.561 5 , P I O.OI), irrespective of the underlying disease state. Further