Streptococcus mutanssubspeciessobrinusserotyped(B13) was cultivated in a fermentor under controlled conditions in two different media: a complex proteose‐peptone medium and a defined minimal medium (C4). Specimens from different growth phases were examined by electron‐microscopy and tested electron‐immunohistochemically. Irrespective of the growth medium used, no differences were observed in the immunohistochemical staining pattern of bacteria during the lag, the exponential and the early stationary phases. Specimens obtained several hours after exponential growth contained areas where the bacteria showed staining that ranged from a strong deposit of reaction product to no deposit. This appearance seems partly to explain the differences in the intensity of immunohistochemical staining of certain bacteria observed in dental plaques stained for identification ofS. mutanssubspeciessob