**3.2 Flow cytometry and fluorescent probes**

During last decades many fluorescent probes have been used for the semen assessment. The fluorescence of these compounds may be estimated using fluorescent microscopy or flow cytometry. Flow cytometry enables the observation of physical characteristics such as cell size, shape, and also any component or function of the spermatozoon that can be detected by a fluorochrome or fluorescently labeled compound. The analysis is objective and accurate. The great number of spermatozoa (>10 000) can be analyzed in a small volume of samples in a short time. This is considerably more than the total of 200 cells generally observed by microscopic analysis. Thus, the analysis of events detected on dot plots gives the accurate and high reliable results (Peña et al., 2001). It is a sensitive method of detection of subtle differences among spermatozoal populations that may not be detected with other techniques.
