Cellular differentiation is a characteristic that defines normal and neoplastic tissue. Epithelial tumors express keratins that are quantitatively and qualitatively different from normal tissue. Therefore, a study was designed to test the hypothesis that native cellular fluorescence (NCF) can be used to detect in vivo differentiation changes associated with higher keratin content in epidermal thick skin and callus when compared with non-keratinized forearm skin. Nineteen control subjects (males, N equals 12; females, N equals 7) were tested using a xenon based fluorescence spectrophotometer (Mediscience Corp.). Seven emission and five excitation spectra were recorded from two sites on each subject: the ventral forearm and palmar hand at the metacarpal-phalangeal joint. Wavelength ratios were used to quantitate three emission and three excitation scans. Two sample t-test analysis was performed for forearm (FA) vs. hand thick skin (HTS) and callus (CAL). The emission scans ((lambda) Ex 300 nm, (lambda) Em 320 - 580 nm; (lambda) Ex 340 nm, (lambda) Em 360 - 660 nm; (lambda) Ex 365 nm, (lambda) Em 400 - 700 nm) are statistically significant (P less than 0.0001) at the wavelength ratios 340 nm/440 nm, 420 nm/540 nm, and 440 nm/540 nm, respectively. The excitation scans ((lambda) Ex 200 - 320 nm, (lambda) Em 340 nm; (lambda) Ex 200 - 360 nm, (lambda) Em 380 nm; (lambda) Ex 270 - 500 nm, (lambda) Em 520 nm) are statistically significant (P less than 0.0001) at the wavelength ratios 260 nm/290 nm, 290 nm/340 nm, and 320 nm/345 nm, respectively. In vivo tissue fluorescence can be used to detect the influence of keratin on the spectral profiles of the epidermis.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.