Biochemical and morphological examinations on human semen have been carried out on more than 2000 patients at a fertility clinic, and the findings are reported in this paper. The fructolysis of human semen depends on the presence of normal spermatozoa; centrifuged cell-free seminal plasma shows no fructolysis. The relationship between fructose content and the sperm count of the semen has been investigated. It was found that the absolute fructose concentration was inversely proportional to the number of spermatozoa. The normal fructose value was found to be 1200 to 4500 [mu]g/ml; values below 1200 [mu]g/ml were considered to be pathological and due to inflammation of the seminal vesicles or to androgen deficiency. The variation in fructose content in a normal patient (1200 to 4500 [mu]/ml) was never found to be below 1200 [mu]g/ml. The fructose concentration was diminished following abstinence of 8 to 14 days or more. The average fructose value is higher in groups of young patients than in groups of aged patients. Particular attention was given to the group of patients with "normospermia with postpubescent interstitial-cell insufficiency" in this investigation. These patients are infertile but after treatment with methyltestosterone or chorionic gonadotrophin the fructose value returned to normal and the patients became fertile.