The utilization of urea nitrogen was examined in 10 healthy adult men from a village near Lufa, in the Eastern Highlands Province of Papua New Guinea. The staple diet of these men was sweet potatoes. [15N]urea was used as tracer for urea released into their intestinal tracts and the utilization of the urea-N was estimated from the trend of 15N. The men were orally given [15N]urea at the beginning of the study and then their daily protein intake, serum protein levels, 15N excretion in the feces and urine, 15N retention in the whole body and 15N incorporation into serum protein were examined. Their daily protein intake (32.2 +/- 8.6 g/day) was low, but their serum protein level (8.05 +/- 0.41 g/100 ml) was within the normal range. 15N retention in the whole body on day 3 was estimated to be 35.4 +/- 20.2% of the total amount administered, calculated from the recoveries in the feces (1.64 +/- 0.85%) and urine (63.0 +/- 20.5%) on days 1-3. The utilization of urea nitrogen in Papua New Guinea highlanders was confirmed from the finding of 15N incorporation into serum proteins on day 3 (0.008 +/- 0.005 atom% excess). This incorporation was negatively correlated with the urinary nitrogen excretion and serum protein level. This correlation suggests that Papua New Guinea highlanders with low urinary nitrogen excretion or a low level in serum protein, who are in a poor state of protein nutrition, tend to utilize more urea nitrogen for the synthesis of serum protein.