Recent advances in theoretical and practical aspects of the application of fluorescence cytophotometry to biological specimens were described. A new method of quantitative detection of DNA damage by computer analysis of Feulgen hydrolysis curve was introduced. The rate constans of depurination and depolymerization in Feulgen hydrolysis can be expressed in the form of k=AN2·exp (-E/RT), where A is a constant, N is HCl concentration, E is the activation energy, R is the gas constant, and T is the absolute temperature. And the Feulgen hydrolysis curve is determined by y (t) =y0k1/ (k2-k1) · (ek1t-ek2t), where y (t) is the amount of apurinic acid stainable with Feulgen reaction at hydrolysis time t, y0 is the amount of apurinic acid theoretically present at t=0. Computer fitting of Feulgen hydrolysis curve to these function makes it possible to determine the values of y0, k1, k2, and E. 1/k1 correlates with the degree of chromatin condensation, k2 reflects the degree of DNA instability, and y0 corresponds to the yield of single-stranded DNA induced by DNA damage due to aging, chemical carcinogen, radiation etc. The values of y0 and k2 are larger for the nuclear DNA in cancerous cells than in noncancerous cells. The increased DNA instability in cancerous cells made it possible to differentially stain cancerous cells with acridine orange after acid hydrolysis. This new technique is a useful tool for detecting malignancy in exfoliative cytology and malignant transformation by chemical carcinogen treatment. Some applications of fluorescence cytophotometry on viable cells were also described.