A latex particle agglutination assay was developed to detect type III group B streptococcal antigen in cerebrospinal fluid. Agglutination was observed in each of 12 initial and 54% of 26 subsequent CSF specimens obtained a mean of 53 hours after the institution of antimicrobial therapy. Latex agglutination was more sensitive than countercurrent immunoelectrophoresis for the detection of type III group B streptococcal antigen in CSF from neonates and young infants with meningitis.