Meningococci and Haemophilus influenzae may invade the subarachnoid space during the bacteriaemic phase without impairment of the blood-CSF barrier and in the absence of any leucocyte reaction. In pneumococcal meningitis the CSF may also contain less than 100 cells/microliter despite the presence of "pure bacterial cultures", but the barrier is completely broken when the serum/CSF concentration ratio is below 10. A clinical analysis of eight patients with fewer than 100 cells/microliter revealed that the first symptoms of meningitis appeared at least 3 days prior to the diagnostic lumbar puncture. There was a strong neutrophilic reaction in the blood with a prevalence of juvenile forms in most cases, indicating intact antibacterial defence mechanisms. Within 24 h after the start of antibiotic therapy the cell number rose above 2000/microliter accompanied by disappearance of pneumococci. Six of the eight patients died. In three cases autopsy revealed thick layers of pus over the convexities, indicating a compartmental separation of the ventricles and the spinal subarachnoid space. In one case of late diagnosed bacterial meningitis with a pleocytosis of 430/microliter the CSF lysozyme level was seven times higher than compatible with this cell number. Hyperphagocytosis and cellular disintegration is thought to cause the leucopenia within the spinal CSF compartment. "Apurulent bacterial meningitis" can be seen as a disease entity that is a diagnostic pitfall and also a prognostic sign.