Inflammatory influx of leucocytes and granulocytes into the lung, histopathological consequence and lung bacterial burden after infection with S. pneumoniae. Mice transnasally infected with S. pneumoniae were killed 12 hr, 24 hr or 48 hr after infection. Control mice were challenged with 20 μl PBS, and killed after 48 hr. Lung cells were counted using a haemocytometer and leucocytes were further differentiated by FACS analysis of CD45 and Gr-1 expression. Data are presented as mean leucocyte (a) or granulocyte (b) numbers ± SEM, *P < 0·05, **P < 0·01 versus control, n = 5 each. The representative haematoxylin & eosin-stained cryosection of a blood-free mouse lung 48 hr after pneumococcal challenge confirms severe pneumonia (c). For the quantification of bacterial load, mice were killed 12 hr, 24 hr or 48 hr after infection. Blood-free lungs were homogenized, plated on blood agar and incubated at 37° for 16 hr before colony counting. Data are depicted as log colony forming units (CFU) ± SEM, **P < 0·001 versus control (non-infected mice), n = 5 each (d).