This study assessed in vivo and in vitro parameters of cell-mediated immunity in 45 tuberculous meningitis patients. Delayed-type hypersensitivity responses to purified protein derivative of tuberculin (PPD), Candida albicans and streptokinase-streptodornase antigens were lacking in 34 patients compared to 2 of 54 normal volunteers. Circulating T lymphocyte subpopulations (E-rosetting active and total T cells) were diminished in 41 patients studied compared to 41 normal donors (p less than 0.001). Lymphocyte blast responses to PPD (25 micrograms-500 micrograms/ml) were positive but quantitatively depressed in all 34 patients studied compared to PPD positive normal donors (p less than 0.001). Phytohemagglutinin responses were significantly depressed (p less than 0.001). Repeat studies in 20 patients 4-5 months post-therapy resulted in reversals of all parameters to control values with heightened blastogenic responses to all PPD concentrations.