Intravenously injected 125I-bovine superoxide dismutase in mice is essentially cleared from organs in 24 h. The 125I activity per g of tissue at 1 h varies among organs with the brain and kidneys showing the lowest and highest levels, respectively. Within tissues, degradation occurs so that, at 1 h after administration, 5-74% of the 125I activity remains associated with the enzyme, depending on the tissue. A small amount of the labelled enzyme is found in washed cells of thymus, lung, spleen, liver and brain.