beta-Glucuronidase and carcinoembryonic antigen (CEA) were measured in the cerebrospinal fluid of patients with cancer. Both substances were found to reliably detect the presence of leptomeningeal infiltration by carcinoma. Neither substance was reliable in the detection of leptomeningeal infiltration by lymphoma or of metastases to the brain parenchyma or spinal epidural space. beta-Glucuronidase was moderately elevated in chronic infectious meningitis, whereas CEA was not. Both markers approached control levels with favorable treatment of the leptomeningeal metastases, reflecting the effectiveness of treatment. Both beta-glucuronidase and CEA hold promise as indicators of early metastatic involvement of the leptomeninges by carcinoma.