A method of isolating X-chromosome bearing sperm cells and Y-chromosome bearing sperm cells, comprising the steps of: (a) collecting sperm cells from a male of a species of mammal; (b) introducing said sperm cells into a fluid stream; (c) determining a sex differentiation characteristic of said sperm cells entrained in said fluid stream by: staining an amount of DNA within the nucleus of said sperm cells; irradiating said stained DNA within said nucleus of said sperm cells; detecting fluorescent light emitted from said irradiated stained DNA within said nucleus of said sperm cells wherein said step of detecting fluorescent light emitted from said irradiated stained DNA within said nucleus of said sperm cells comprises generating a signal with a photomultiplier tube and assessing an amount of DNA within the nucleus of said sperm cells; (d) forming a plurality of droplets each having one of said sperm cells entrained; (e) charging each of said droplets differentially based upon said sex differentiation characteristic of said sperm cells entrained in said droplets; (f) separating differentiated sperm cells into a X-chromosome bearing population and a Y-chromosome bearing population by deflecting each of said droplets; and (g) producing separate X-chromosome bearing and Y-chromosome bearing populations of sperm cells by differentially collecting each of said droplets based upon said sex differentiation characteristic of said sperm cells entrained in said droplets; said method being characterized in that during said step of detecting fluorescent light the operating voltage of said photomultiplier tube is reduced below the lower limit of linear operation of said photomultiplier tube. (a) collecting sperm cells from a male of a species of mammal; (b) introducing said sperm cells into a fluid stream; (c) determining a sex differentiation characteristic of said sperm cells entrained in said fluid stream by: staining an amount of DNA within the nucleus of said sperm cells; irradiating said stained DNA within said nucleus of said sperm cells; detecting fluorescent light emitted from said irradiated stained DNA within said nucleus of said sperm cells wherein said step of detecting fluorescent light emitted from said irradiated stained DNA within said nucleus of said sperm cells comprises generating a signal with a photomultiplier tube and assessing an amount of DNA within the nucleus of said sperm cells; staining an amount of DNA within the nucleus of said sperm cells; irradiating said stained DNA within said nucleus of said sperm cells; detecting fluorescent light emitted from said irradiated stained DNA within said nucleus of said sperm cells wherein said step of detecting fluorescent light emitted from said irradiated stained DNA within said nucleus of said sperm cells comprises generating a signal with a photomultiplier tube and assessing an amount of DNA within the nucleus of said sperm cells; (d) forming a plurality of droplets each having one of said sperm cells entrained; (e) charging each of said droplets differentially based upon said sex differentiation characteristic of said sperm cells entrained in said droplets; (f) separating differentiated sperm cells into a X-chromosome bearing population and a Y-chromosome bearing population by deflecting each of said droplets; and (g) producing separate X-chromosome bearing and Y-chromosome bearing populations of sperm cells by differentially collecting each of said droplets based upon said sex differentiation characteristic of said sperm cells entrained in said droplets; said method being characterized in that during said step of detecting fluorescent light the operating voltage of said photomultiplier tube is reduced below the lower limit of linear operation of said photomultiplier tube.