Two enzymes of the haem biosynthetic pathway were investigated in patients with variegate porphyria. Protoporphyrinogen oxidase in cultures of Epstein-Barr virus transformed lymphoblasts from twenty-seven patients showed a mean maximal velocity (Vmax) of 0.39 +/- 0.08+ nmol of protoporphyrin mg protein-1 h-1, a 52% reduction (P less than 0.001) from a non-porphyric control group (0.82 +/- 0.10). Km values (1.00 +/- 0.27 microM) did not differ significantly (P greater than 0.05) from control values in any of the patients. The mean Vmax of porphobilinogen deaminase in the cultures was 1.50 +/- 0.18 nmol of uroporphyrin mg protein-1 min-1, a 24% reduction (P less than 0.001) from controls (1.94 +/- 0.14). Mean porphobilinogen deaminase activity in the erythrocytes of twenty-one patients with variegate porphyria was 8.37 +/- 1.99 nmol of uroporphyrin 1 erythrocytes-1 s-1, a 28% reduction (P less than 0.001) from normal (11.98 +/- 2.11). The reduced activities of these two enzymes comply with the expression of variegate porphyria during its quiescent and acute phases.