Serum alkaline phosphatase isoenzymes were determined quantitatively by electrophoresis on cellulose acetate in 168 patients with rheumatic diseases subgrouped for disease activity. Median values of total alkaline phosphatase and bone isoenzyme activity, as well as frequency of patients showing pathological values, increased gradually and significantly corresponding to disease activity in rheumatoid arthritis and ankylosing spondylitis, from 0% in inactive to 90% in very active forms. Bone isoenzyme was much more sensitive than total alkaline phosphatase in moderate disease activity and was also correlated to the number of involved extravertebral joints and pain in ankylosing spondylitis. No correlation was found with stage or duration of disease, age, sex, and erythrocyte sedimentation rate. Additional to bone isoenzyme, liver isoenzymes were elevated in some patients, but with only a weak correlation with disease activity. The intestinal isoenzymes were always normal. We conclude that quantitative determination of serum alkaline phosphatase bone isoenzyme activity is a major indicator for the assessment of disease activity and therapeutic monitoring in rheumatoid arthritis and ankylosing spondylitis.