Histologic and ultrastructal studies of heart, lung, liver and kidney were made of mice treated with interleukin-2 singly and in combination with interleukin-1 and or interferon-alpha. The vascular leak syndrome induced by interleukin-2 was ameliorated by concomitent treatment with interleukin-1. This reduction in toxicity may be useful in the treatment of neoplasms with interleukin-2.