Sixteen out of eighteen meningeal carcinomas (89%) secreted carcinoembryonic antigen (CEA) into the cerebrospinal fluid, where it could be quantified separately from the portion originating from the circulating blood. The discrimination of both fractions was performed according to an empirical approach analogous to the immunoglobulins. Only 47% of the intraparenchymal carcinomas released CEA into the CSF compartment and it is possible that the extra-cellular space of these tumour sites does not communicate with the free CSF space. Extradural metastases may cause an impairment of the blood-CSF barrier via restrictions of the CSF fluid turnover, but the dura remains impermeable for the tumour marker. Seven out of 54 primary brain tumours (13%) released carcinoembryonic antigen into the cerebrospinal fluid.