Suppressible and non-suppressible insulin-like activities (ILA) of plasma from 3 patients with congenital generalized lipodystrophy have been studied, employing isolated fat cells from rat epididymal adipose tissue. 2. In all 3 patients the fasting ILA was markedly increased compared with the normal controls. In one of the patients (I.T.) total ILA rose to about 800 muU per ml during an iv glucose tolerance test. 3. The observed total ILA was in all cases (controls included) equal to or slightly higher than the previously determined immunoreactive plasma insulin (IRI). The exact determination of ILA was, however, hampered by a dilution effect, which was present even at high plasma dilutions. 4. In 2 of the patients addition of insulin antiserum inhibited plasma ILA by about 50%. In the third patient (I.T.), who exhibited the highest insulin level, at least 85% of the activity was suppressed by insulin antibodies. The levels of non-suppressible ILA were higher than in the controls in terms of muU per ml, but lower than in the controls when related to total ILA. 5. These findings strongly support our previous conclusion that the elevated plasma insulin seen in congenital generalized lipodystrophy is mainly due to true pancreatic insulin. 6. Since the effect of plasma insulin on isolated fat cells were freely expressed, i.e. suppressible ILA was equal to or slightly lower than IRI, the presence of a circulating insulin antagonist in this disease may be excluded.
