The plate count method for quantitative assessment of yeast "viability" (cell proliferation capacity), is the method still generally used by microbiologists (CFUs). In the present communication, we describe the results obtained using FCM to evaluate cytotoxic effects of acetic acid (AA), at final concentrations of 1% (only in some experiments), 1.5% and 3%, on Saccharomyces cerevisiae and Zygosaccharomyces bailii, after different times of incubation (7 times), in mineral medium with glucose, at 25 °C, and at pH=3.0. For this purpose we double stained both yeast cells species with fluorescein diacetate (FDA) and propidium iodide (PI), and in the same conditions with the new fluorochrome FUN-1 (Molecular Probes). With the double staining technique, FDA(-) cells and PI (+) cells were considered "nonviable" cells. "viable" cells loaded with FUN-1, process this fluorochrome concentrating it within cell vacuoles, forming there red fluorescent structures. In "nonviable" cells it stays in the cytosol and is responsible for a diffuse red fluorescence, with a mean intensity that is higher in these cells, compared to the viable cells. Plots defined by the % of "nonviable" cells against time gave us the kinetics of the alterations induced by acetic acid. Results show that I) different values are obtained with different markers and/or methods in each yeast (obviously at early times) and that II) each yeast specie has a different "sensitivity" to acetic acid treatment. To illustrate these facts, the % values of "nonviable" cells of S. cerevisiae, found after 40 minutes of incubation with acetic acid (AA), at different concentrations, in 5 experiments, were selected and are the following: 1) with FDA: 2.04%±0.49% [0% AA, final concentration], 27.39%ñ6.18% [1.5%], and 94.94ñ3.17% [3%]; 2) with PI: 0.45%±0.05% [0%], 13.32%±1.21% [1.5%], and 87.97±6.54% [3%]; 3) with FUN-1: 2.18%±0.32% [0%], 95.30%±2.60% [1.5%], and 95.69±1.82% [3%]; 4) with CFU's: 0% [0%], 93.63%±4.30% [1.5%], and 100% [3%]. The % values of "nonviable" cells of Z bailii found in the same conditions, are the following: 1) with FDA: 35.17%±4.08% [0%], 38.81%±8.94% [1.5%], and 17.9%±2.76% [3%]; 2) with PI: 0.28%±0.11% [0%], 0.77%±0.10% [1.5%], and 11.07%±1.97% [3%]; 3) with FUN-1: 2.39%ñ1.16% [0%], 5.70%ñ3.76% [1.5%], and 45.89ñ4.67% [3%]; 4) with CFU's: 6.73%±2.50% [0%], 40.57%±2.00% [1.5%], and 89.42±3.14% [3%]. The results obtained with FDA in Z. bailii were unexpected. We speculate that an intracellular pH change is interfering with these measurements. We conclude that: I) when we pretend to assess cell "viability" changes, we should bare in mind that different methods (may be with different probes) measure different effects on cell structure(s) and/or cell activity(ies), like cell proliferation II) that the kinetic approach is essential at sub-lethal treatments III) specific differences inherent to particular cell types may interfere with some evaluations IV) presently flow cytometry may be considered an advantageous alternative to achieve cell "functionality" measurements.