Quantification of bacteria in various effusion materials (serous, sticky glue, mucopurulent and purulent) collected from 128 middle ears (96 patients) was performed. After sampling, the effusion material was immediately flushed onto a glass slide, stained with acridine orange and examined under a fluorescence microscope. Quantification of bacteria was performed as described. 93% of the serous (n = 28) and 61% of the sticky glue (n = 46) effusion materials contained no bacteria whatsoever. The infectious sticky glue material (n = 18) contained 2.0 x 10(5) (median value), mucopurulent effusion material (n = 24) 2.6 x 10(7) (median value) and purulent effusion (n = 30) 10(9) (median value) bacteria per ml effusion material. Direct microscopic examination of middle ear effusion material, including quantification of the bacteria involved, provides much information about the pathophysiology of the middle ear inflammation.