Free and conjugated [14C]spermidine were measured in plasma samples from normal individuals and cystic fibrosis patients. Within 4 minutes, the 14C-labeled material in the plasma from normal individuals was 70 percent conjugated compared to no detectable conjugation by cystic fibrosis patients. Further, the patients excreted only 11 to 13 percent of the [14C]spermidine in their urine within 72 hours whereas normal excretion was 60 to 76 percent. In both cases, the labeled material was in a conjugated form.