The mean level of phosphorus was significantly higher in a group of 96 Indonesian patients with pulmonary tuberculosis than in 100 control subjects. The levels in disease showed a moderate correlation with those of the globulin proteins, but not the gamma-globulins. There was an association between levels of phosphorus and those of transferrin but not with those of seven other acute-phase associated proteins. There was a tendency for phosphorus levels to be higher in those with elevated polymorphonuclear leucocyte counts and, although not significantly so, in those with more extensive active disease, suggesting an association with tissue destruction. It appears therefore more relevant to study the behaviour of this element than that of calcium in tuberculosis.