KOVACS reagent, consisting of phosphate-buffered saline (PBS) and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), is widely used in medical and histological diagnostics. The PBS component serves as an ideal medium for preserving cell structures and is frequently used in cell culture, molecular biology and immunostaining. TMPD, on the other hand, plays a crucial role in oxidase assays, especially the Kovacs oxidase assay, for quantifying terminal oxidase activity in bacterial cells. TMPD oxidase activity correlates exceptionally well with the results of the standard Kovacs oxidase assay. This allows precise discrimination between oxidase-positive and oxidase-negative bacteria, which contributes to improved bacterial classification. The use of this reagent therefore provides a robust and reproducible method for the examination and determination of microbial patterns and the assessment of cell behavior in histological specimens.