The activities of pyrimidine 5'-nucleotidase (P5N) and the nucleotide pools in the erythrocytes from 19 members of a Dutch family with P5N deficiency were measured. In the erythrocytes of 5 (out of 6) apparent heterozygotes (based on P5N activities), an increased amount of UDP-N-acetylhexosamines was found. This increase was also found in the erythrocytes of 2 (out of 3) questionable heterozygotes (P5N activity below normal range, but not below normal mean--2 X SD) and not in the erythrocytes of family members with a normal P5N activity nor in erythrocytes from healthy donors. We conclude that analysis of the ribonucleotide patterns, in combination with determination of P5N activity, allows a more accurate diagnosis of heterozygosity for P5N.