Immature monocytes residing m the spleens of newborn nine less than 6–7 days of age are known to suppress T‐dependent; and T‐independent immune responses in vitro and in vivo. Suppression is mediated in part through products secreted In the newborn monocytes, In the present study. we have identified 3 monokines secreted by newborn, but not adult, monocytes correlating with materials possessing significant suppressor activity in vitro. These monokines have molecular weights of 5N. 58. 10.8. and 10 kilodaltons. appear to be proteins, and are insensitive to heat inactivation. The 58 kd material is antigenically distinct from alpha‐fetoprotein. while the lower molecular weight materials arc too large to be prostaglandins. Although these monokines can he shown to have activity in vitro, they are not as efficient as newborn monocytes in preventing alloreactivity