A method is described for estimating the numbers of animal cells in multi-well culture by simultaneously measuring the lactate dehydrogenase activity of the total culture and the medium. The difference between the two reflects the dehydrogenase content of the cells and correlates with cell number. This LDH/INT method was tested using several lines of normal and transformed suspension and adherent cells. The lactate dehydrogenase activities of duplicate cultures were determined colourimetrically using reaction cocktails containing lactate, NAD(+), diaphorase, and p-iodonitrotetrazolium violet, with and without Triton X-100. The difference in absorbance at 490 nm (Delta A(490) = A(490), test - A(490, control)) was used to calculate the lactate dehydrogenase activity of the total culture (+ Triton) and the medium (- Triton). The cellular lactate dehydrogenase activity (difference between total and medium dehydrogenase activities) was proportional to viable cell number. The effects on cell growth of four metabolic inhibitors, sodium azide, actinomycin D, cycloheximide, and taxol, were determined using the LDH/INT assay and direct cell counting. The inhibitor concentrations that caused decreases in the LDH activity and cell number by 50% were similar. The LDH/INT assay is quick and sensitive, works equally well for adherent and suspension cells, and provides information about LDH activities of both the medium and cells. It is particularly useful for screening potential cell-growth inhibitors.
