The concentrations of complement factors C3 and C4 were quantified by single radial immunodiffusion in unconcentrated cerebrospinal fluid (CSF) and in serum from 38 patients up to 2 months after onset of acute aseptic meningitigs (AM). Elevated absolute concentrations were found in CSF in 26 and 35 patients, respectively, and in serum in 8 and 31, respectively. Elevation of the CSF C3 index, equal to (CSF/serum C3):(CSF/serum albumin), and of the corresponding CSF C4 index were found in 16 and 7 patients, respectively, as evidence of intrathecal synthesis. Only minor differences of the frequencies of elevated CSF C3 and C4 indices were encountered over the course of AM up to 2 months after onset. The occurrence of intrathecal C3 and C4 synthesis in AM is proposed as reflecting activation of hitherto unknown significance within the central nervous system.