Patients with active lymphoproliferative diseases (LPD) were shown to have high activity of lymphocyte uroporphyrinogen synthase (L-UROS), the enzyme which converts porphobilinogen to uroporphyrinogen. The mean L-UROS activity of 64 first-degree relatives of patients with LPD was significantly higher than that of a control group and 45% of these relatives had pathological values of L-UROS. L-UROS activity was also determined in the spouses of 2 patients and was pathologically elevated in both. The pattern of pathological values among family members may indicate the presence of a communicable agent.