Humoral immune activities generally decline in both mouse and man as the individual ages. It is not known what role the catabolism of immunoglobulin plays in the decline. To have a better insight into this problem, the catabolic half-lives of radioiodinated immunoglobulin and albumin were assessed in mice 3 to 35 mo. of age. In itial studies on total serum protein, albumin, and immunoglobulin g concentration revealed that they tend to vary with age. The half-life of immunoglobulin g decreased sharply until about 6 mo. of age, followed by a more gradual decrease throughout the remainder of life. On the other hand, the half-life of albumin remained relatively constant until very late in life, when a marked increase was observed.