To understand the thymic hormone production in thymic tumors and the myasthenic thymus, we have studied the concentrations of two thymic hormones (prothmosin alpha and thymosin beta 4) in these tissues. A total of forty four thymus or thymic tumor tissues were obtained from patients who underwent thoracic operation. These tissues consisted of a control group (n = 25) and three study groups including a thymoma group (N = 8), thymic carcinoma group (n = 3) and myasthenic thymus group (n = 8). The age-related curve of the control group, which provided histologically normal thymic tissues for the production of prothymosin alpha and thymosin beta 4, was established. It was found that in the control group the concentrations (microgram/g tissue) of prothymosin alpha and thymosin beta 4 decreased with age, and that the ranges were from 171 micrograms/g to trace and from 243 micrograms/g to trace, respectively. The amounts were highest at puberty and then gradually decreased, and only trace amounts of both polypeptides were present at age 60. Study cases which deviated from the standard curves were identified. Our results indicated that the contents of thymic hormones significantly increased in the tissues of invasive or non-invasive thymoma, decreased in thymic carcinoma, and were not significantly changed in myasthenic thymus as compared with those of the age-related normal thymus tissues. These findings confirm the notion that only thymomas produce significantly higher thymic hormones.