Two hundred and ten patients with meningismus and the infections of the central nervous system (CNS) with the clinical symptoms and signs of the acute serous meningitis syndrome, were divided into groups according to etiology (enterovirus meningitis-ENTERO, serous meningitis various etiology-SM and tuberculosis meningitis-TBC). Intrathecal synthesis (ITS) of C3c and C4 complement components and IgG were determined by the method of cerebrospinal indexes (I), to examine their role in differential diagnosis of this syndrome. Correlative study between the CSF/serum ratio (Q) for albumin (Alb) and QC3c and QC4 in patients with no proven ITS of this two complement proteins, and the comparative study of the increased value of C3cI and C4I (and IgGI) between the examined groups of the patients was done. Highly significant correlations were found between QAlb and QC3c (r = 0.89, p<0.001) and QC4 (r = 0.85, p<0.001). In 22,4% of the examined patients ITS of C3c and C4 were found. There was no difference in frequency of ITS of the two complement proteins between the examined groups, nor inside any particular group. TBC group had significantly lower (p<0.05) intensity of ITS of C3c and C4 than MNG and ENTERO, and significantly higher intensity of ITS of IgG (p<0.05) than the other tested groups. CSF index was confirmed as a valid method to detect intrathecal C3c and C4 production. Determination of ITS C3c and C4 could not be of great help in differential diagnosis in the acute serous meningitis syndrome. The intensity of ITS of C3c and C4, related to the intensity of ITS of IgG, could be of help in the determination of the duration of the disease.