Part IIA: If the DLT is diarrhea in part I, then part IIA is opened. Patients receive oral phenobarbital, cyclosporine as in part I, and irinotecan at the MTD from part I. Dose escalation occurs as in part I to determine a new MTD. If the DLT continues to be diarrhea, the study is closed. Part IIB: If the DLT is neutropenia in part I, then part IIB is opened. Patients receive cyclosporine as in part I and escalating doses of irinotecan to determine a new MTD.