A technique was developed for sectioning fresh red spruce foliage (Picea rubensSarg.) for use in fluorescence microscopy. This allowed rapid examination of mesophyll in 3–5 mm needle sections. Healthy, ozone treated and cold stressed needles were examined to assess the utility of this technique for early detection of damage. Healthy mesophyll cells fluoresced bright red, while injured cells fluoresced yellow-green in ozone treated needles, and yellow-orange in frozen needles. Shifts in fluorescence wavelengths may be useful for early detection of injury to mesophyll before it is evident by standard light or electron microscopy.