Plasma insulin has been characterized by its biologic, immunologic, physical, and chemical similarity to pancreatic insulin. A higher molecular weight heterogenous component (peak "b") of pancreatic insulin that contains the biosynthetic precursor of insulin (proinsulin) has recently been described. Plasma insulin as measured by radioimmunoassay is composed of at least two components. Filtration of plasma on G-50 Sephadex yields a major component ("little" insulin) which is indistinguishable from the major component of pancreatic insulin and a second component "big" insulin which is less retarded by the gel similar to peak "b." Both components react strongly with insulin antisera and are secreted by the pancreas in response to stimuli of insulin secretion. The characteristics of these recently described components of pancreatic and of plasma insulin are described and discussed.