Sera from multiple sclerosis patients show a deficit of intestinal alkaline phosphatase (serum type Pp2) by comparison with normal sera. This is not due to variation in ABO frequency, since the specimens from patients and normals are matched for ABO frequency, and it is not due to differences in secretor frequency, but represents a real dificit for Pp2 in patients with multiple sclerosis, particularly noticeable in group 0 individuals.