Eosinophilia of the cerebrospinal fluid (CSF) in permissive (rats) and non-permissive (mice) hosts infected with Angiostrongylus cantonensis, and the possible mechanism of the eosinophilia were studied. In three strains of thymic mice (ICR, ddY and BALB/c), the infection provoked a marked CSF eosinophilia starting at around day 12, reaching a peak level at day 20 and maintaining significantly high levels until day 35. In contrast, in athymic nude mice of BALB/c strain the infection failed to evoke this eosinophilia, suggesting T-cell dependence of murine CSF eosinophilia. Humoral antibodies did not correlate with the induction of eosinophilia. A time-course study of worm recovery in the mouse brains indicated a gradual but consistent reduction in worm burden in accordance with the rapid rise in CSF eosinophil levels. Bone marrow eosinophilia occurred in mice at day 5, which preceded CSF eosinophilia. Jirds, a permissive but less susceptible host, developed a CSF eosinophilia with a peak level at day 17, but which declined rapidly following the peak. Permissive rat hosts developed significant peripheral and bone marrow eosinophilia at day 35 but their CSF eosinophilia was markedly less prominent than that of mice and jirds. These data clearly indicate that there are distinct differences in the mechanism of eosinophilia and eosinophilia-inducing factors between permissive and non-permissive hosts.