We studied the effects of exogenously administered recombinant murine interleukin (IL)-10 on the development of experimental autoimmune orchitis (EAO) in C3H/He mice. IL-10 significantly augments histological signs of EAO when administered for 6 consecutive days from days 15 to 20 after primary immunisations with testicular germ cells. These data demonstrate that IL-10, in addition to its well-known antiinflammatory property, also has proinflammatory functions capable of up-regulating testicular immunoinflammatory processes in vivo.