Hematological values and serum amino-acid concentrations were measured in 17 healthy male adult Papua New Guinea highlanders who live on a sweet-potato staple diet. Hematological values were within the normal range, except for a low serum urea concentration. The concentrations of serum threonine, valine, isoleucine, leucine and tyrosine were significantly lower, and those of arginine, glutamate, glycine and alanine were significantly higher, than in Japanese controls. These amino acid patterns in the serum of Papua New Guinea highlanders are an indication of low protein nutrition and adequate energy supply. Some essential amino acid and urea concentrations in the serum of nine Papua New Guinea subjects fed on an adequate protein diet (1.3 g/kg body weight, about twice their habitual diet) for 13 days were significantly increased but were still significantly lower than those of Japanese subjects. Serum alanine decreased on an adequate protein diet. These results show that amino acid uptake and utilization by peripheral tissues may be accelerated on an adequate protein diet. Blood status and serum amino acid concentrations did not show any change except for urea and some amino acids, when five Papua New Guinea highlanders were fed on a low protein diet (0.6 g/kg) consecutively for 13 days.