The rates of catabolism of human gammaG-immunoglobulins of subclasses gammaG(1), gammaG(2), gammaG(3), and gammaG(4) were studied by determining the rates of elimination from the circulation of pairs of (131)I-and (125)I-labeled gammaG-myeloma proteins in 57 patients suffering from cancer other than multiple myeloma. On the average, gammaG(1)-, gammaG(2)-, and gammaG(4)-myeloma proteins were catabolized at a rate similar to that of normal gammaG-immunoglobulin, whereas gammaG(3)-myeloma proteins were catabolized more rapidly than normal gammaG-immunoglobulin. The average half-lives for the myeloma proteins were 12.3 days for normal gammaG, 11.6 days for gammaG(1), 12.4 days for gammaG(2), 8.2 days for gammaG(3), and 11.3 days for gammaG(4). However, significant differences in catabolic rates were observed when individual myeloma proteins of a single subclass were compared. These individual variations were present within all four heavy chain subclasses. The extent of differences ranged from 10 to 47%. The catabolic rate of normal gammaG was in an intermediate range when compared with myeloma proteins of relatively long and short half-lives. The rate of catabolism of an individual myeloma protein did not correlate with its light chain type, Gm factor, carbohydrate content, or electrophoretic mobility. These findings indicate that the structure(s) related to the catabolism of gammaG-immunoglobulins are complex and differ from one immunoglobulin molecule to another.