Lactococci are coccoid Gram-positive, anaerobic bacteria which produce l(+)-lactic acid from lactose in spontaneously fermented raw milk which is left at ambient temperatures around 20–30°C for 10–20 h. They are commonly called ‘mesophilic lactic streptococci’. It is tempting to suggest that the first isolation, identification and description of the chemical entity lactic acid by Carl Wilhelm Scheele from sour milk in Sweden in the year 1780, was actually l (+)-lactic acid produced by lactococci. The microbial nature of lactic fermentation was recognized in 1857 by Louis Pasteur. The first bacterial pure culture on earth, obtained and scientifically described by Joseph Lister (1873) was Lactococcus lactis, at that time called: ‘Bacterium lactis’. Admitting then that we had here to deal with only one bacterium, it presents such peculiarities both morphologically and physiologically as to justify us, I think, in regarding it a definite and recognizable species for which I venture to suggest the name Bacterium lactis. This I do with diffidence, believing that up to this time no bacterium has been defined by reliable characters. Whether this is the only bacterium that can occasion the lactic acid fermentation, I am not prepared to say.