1. Colony-stimulating activity appeared in the perfusate of the isolated rat liver during perfusions with either whole rat blood, rat plasma or an artificial perfusate of Eagle's medium and albumin.
2. Dialysable inhibitors of colony formation were also released during perfusions.
3. Colony-stimulating activity in artificial perfusate could be enhanced by the addition of rat plasma in vitro. Concentrations of cycloheximide that inhibited albumin synthesis by the liver did not inhibit the release of colony-stimulating activity.