The standard plate count (SPC-32 C) and the direct microscopic count (DMC) of samples of commercially pasteurized milk inoculated with pure cultures of actively growing (18–24 hr growth) bacteria commonly found in milk were compared. Four staining procedures for DMC were used: (a) Levowitz-Weber's methylene blue stain; (b) a modified Levowitz-Weber stain incorporating basic fuchsin; (c) alcoholic-acetic acid fixation followed by periodic acid-bisulfite treatment and staining with pH4 toluidine blue; and (d) alcoholic-acetic acid fixation and staining with pH4 toluidine blue. Counting was standardized by the use of a geometrical pattern. Correlations between SPC and each DMC procedure, or among the DMC procedures were little influenced by the number of microscopic fields counted, their location on the smear or the definition of “clumping” used. Correlations were influenced by the type of bacterial culture inoculated in milk and by the staining procedure. Precision of DMC was shown to be independent from the staining procedure, but varied directly with the number of cells per field and inversely with the square root of the number of fields counted.

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