Activities of the lysosomal enzymes, cathepsin Bl (CB1), 0- glucuronidase, and /7-yV-acetyl-D-glucosaminidase, as well as sialyl transferase, alkaline phosphatase, and placenta-like alkaline phosphatase, were determined on blind-coded serums from 99 women exposed to diethylstilbestrol (DES) in utero and 40 unexposed subjects of comparable age range. Cathepsin Bl averaged 100%, 1040% (P<0.001), 2720% (P<0.001), and 4760% (P<0.001) of controls in DES-exposed women with no genital tract abnormalities (N=11), adenosis (N=68), adenosis with concomitant dysplasia (N=IS), and clear-cell adenocarcinoma (N=5), respectively. The latter two groups also exhibited modest increments in serum sialyl transferase activity (P<0.01). Activities of the other four enzymes in serums of DESexposed women were unchanged from those of controls, suggesting that alterations in CB1 were not due to generalized increases in lysosomal membrane instability or other gross cellular damage. In 2 DES-exposed women with clear-cell adenocarcinoma, from whom serial samples were available, preoperative levels of serum CB1 fell from a mean of 4280% to values indistinguishable from controls by 7-12 days after tumor excision, concurrently with objective signs of remission. Recrudescence of serum CB1 levels preceded by at least 3 months clinical evidence of persistent adenosis accompanied by vaginal dysplasia. Although the nature of the increments in CBl-like activity in the majority of subjects with DES-related pathology remains to be determined, the findings may complement present methods of physical diagnosis and prognosis.