Red fluorophores synthesized by oral bacteria are important for fluorescence-based diagnosis and treatment because they are used as markers for bacterially infected tissue, mature plaque or calculus. A range of porphyrins has been identified as the source of this fluorescence in carious tissue. It is not clear which of these porphyrins are produced by individual oral bacteria or whether this ability depends on other factors. This study examined and compared the fluorescence spectra produced by selected cultured oral bacteria when grown on agars containing different nutrients with spectra for protoporphyrin IX, Zn-protoporphyrin IX, haematoporphyrin and haematin. Actinomyces israelii (DSM, Deutsche Sammlung von Mikroorganismen, 43320), Actinomyces naeslundii (DSM 43013), Fusobacterium nucleatum (DSM 20482), Lactobacillus casei (DSM 20011), Prevotella intermedia (DSM 20706), Streptococcus mutans (DSM 20523), Streptococcus oralis (DSM 20627), Streptococcus salivarius (DSM 20560) and Streptococcus sobrinus (DSM 20742) were rehydrated and grown anaerobically on caso, caso blood (containing 5% sheep blood) and caso chlorophyll (containing 5% spinach extract) agar for 3 days at 37 degrees C, in the dark. Colonies were harvested, transferred to ethanol, and centrifuged. Fluorescence emission spectra were recorded from the supernatant at 405nm excitation (Fluorolog 3-22. Jobin Yvon-Spex ISA, Edison, NJ, USA). All streptococci, L. casei and F. nucleatum produced red fluorescence when grown on caso and caso chlorophyll agar, but not on caso blood agar. A. naeslundii and P. intermedia emitted intense red fluorescence when grown on caso or caso blood agar but not on caso chlorophyll agar. Fluorescence emission spectra of A. naeslundii and P. intermedia grown on caso blood agar correlated exactly with both fluorescence peaks for protoporphyrin-IX at 632 and 701nm. Most peaks observed could be correlated with at least one of the emission-peaks of protoporphyrin IX, Zn-protoporphyrin IX or haematoporphyrin. Oral bacteria emitted red fluorescence matching known porphyrins, but this depended on nutrients available in the agar.