1. Ten subjects with acute intermittent porphyria from three different families, and 92 relatives, were investigated for their erythrocyte uroporphyrinogen I synthase (EC 18.104.22.168) activities by the spectrofluorimetric method described and for their urinary concentrations of δ-aminolaevulinic acid and porphobilinogen.
2. The mean uroporphyrinogen I synthase activity in 41 healthy women and 41 healthy men showed a significant (P < 0·001) sex difference.
3. A reduction of about 32% of the enzyme activity was observed in the porphyric subjects as compared with values in healthy normal subjects and the values from the porphyric subjects overlapped those of the reference subjects.
4. With the values from the normal subjects in each family used as reference, however, the enzyme activity in normal subjects was twice that in affected subjects. Thus by using an internal family reference uroporphyrinogen I synthase values became more reliable in disclosing latent cases of the disorder. Furthermore, these measurements were shown to have a stronger discriminative power than urinary δ-aminolaevulinic acid and porphobilinogen determinations.