Cephalexin is a medical drug administrated commonly for the treatment of infections.Two methods were examined for determination of cephalexin based on conventional and derivative UV absorption spectrophotometry.The calibration curve at 262 nm has linear range extended from (1.5-100) ppm with relative standard deviations for the slope of 0.91 and correlation coefficient of 0.9993. The effect of pH on the determination of cephalexin at this wavelength was also investigated. The best pH for its determination was found to be 6.7.The effect of the presence of some metal ions that found in human body on the cephalexin determination was studied. We noticed that there are interference between metal ions and cephalexin measurements. The metal ions Na+, K+, Mg2+, Ca2+ were found that have no interference but Cu2+, Fe3+ can interfere with cephalexin.Derivative absorption spectrophotometry was developed for determination of cephalexin in the presence of some metal ions that interfere with cephalexin.First, second, and third derivative spectra at 317 nm, 337 nm and 291nm, respectively for Fe3+, were used for this purpose.First, second, and third derivative spectra at 285 nm, 238 nm and 222nm, respectively for Cu2+, were used for this purpose.The calibration curves for the first, second and third derivative spectra have a linear rang (2-50) ppm, (2-50) ppm and (2-40) ppm with relative standard deviation for the slope of 1.38, 0.32 and 3.47 with correlation coefficient of 0.9993, 0.9996, 0.9990, respectively, for cephalexin determination in the presence of Fe3+. The calibration curves for the first, second and third derivative spectra have a linear rang (2-50) ppm, (2-50) ppm and (2-40) ppm with relative standard deviation for the slope of 3.22, 0.57 and 4.47 with correlation coefficient of 0.9992, 0.9998, 0.9994, respectively, for cephalexin determination in the presence of Cu2+.No effect of pH on the determination of cephalexin using derivative methods was found.