(1) Fetal thymuses, organs from patients who died from diseases that are not clinically known to be associated with concomitant lymphoid tissue involvement, as well as thymuses from patients dying from diseases which affect the lymphatic complex of the body, one way or another, have been investigated for their alkaline phosphatase activity, using Gomori technique and applying four different phosphate esters as substrates. (2) Three substrates (/3-glycerophosphate, riboflavin 5-phosphate and adenosine triphosphate) showed essentially the same pattern of activity in which the cortex and Hassall’s corpuscles were reactive, while the medulla was negative. A reversal of this pattern was demonstrated with 5-monophosphoric acid. (3) Before the age of 32–36 weeks of intra-uterine life there is no alkaline phosphatase activity in the thymus; thereafter, the enzyme begins to make its first appearance. (4) There is a definite increase in the intensity of the reaction with advance of intra-uterine life. This increase in phosphatase content is continued postnatally, to reach its maximum at about the age of 10 years; after that, the enzyme activity gradually subsides. (5) There is a tremendous augmentation of phosphatase activity in the case of diseases which are known to affect the lymphoid complex. (6) The phosphatase activity of the thymus has been discussed in relation to the prevailing concepts about the function of the thymus, with special emphasis on a possible association with ‘lymphocyte-stimulating factor’ production and/or secretion.