As a modification of the classical complement fixation test, a new test for immunodiagnosis in cerebrospinal fluid (CSF) was developed. When the assay was used for the diagnosis of neurocysticercosis in 149 CSF samples from patients and 1036 from controls, results showed 93% concordance with positives and 97% with negatives when compared with the results obtained by ELISA; in addition, it was positive in 12 of 16 cases of neurocysticercosis in which negative results had been obtained by ELISA. The new complement fixation test is particularly useful in CSF because of the immunological peculiarities of the subarachnoid space, where local synthesis of oligoclonal antibodies is induced by infectious agents. Reagents used for the assay are easy to produce and preserve, and the test is inexpensive, reliable and easy to perform. As this test detects the immunobiological consequence of the antigen-antibody reaction, it can be used in conjunction with other assays, such as ELISA, that measure the reaction directly, thus increasing the diagnostic possibilities. It may also be used where financial or technical limitations hinder access to other immunodiagnostic tests.