The effect of zidovudine on beta 2-microglobulin and neopterin in the cerebrospinal fluid (CSF) was studied in ten HIV-seropositive neurologically healthy individuals. CSF concentrations of neopterin decreased by 53% (from 20.3 to 9.5 nmol/l, p < 0.01) and beta 2-microglobulin by 36% (from 2.8 to 1.8 mg/l, p < 0.01) four to eight months after initiation of therapy.