Photochemically induced dynamic nuclear polarization (photo-CIDNP) of proteins requires the introduction of ca. l-7 W of 488 nm light directly into NMR samples (1). This has been previously accomplished by a number of configurations: directing the light beam via an optical bench and mirrors through the bottom of the supercon NMR probe to a flat-bottom NMR sample tube; or, alternatively, focusing the light through a cylindrical quartz rod with a 45” polished end to convey a diverged light beam to the side of a spinning sample tube in the rf coil (2). In both cases, costly probe modifications are necessary, including removal or alteration of the variable temperature control apparatus. Laser beam alignment (into a mirror at the magnet base) can also be awkward and time consuming. Lerman and Cohn (3) illuminated samples by inserting a coupled optical fiber directly into the NMR sample solution. Kaptein and Dykstra (personal communication) have fixed a prism at the end of an optical fiber and situated it between the rf coil windings of a modified Bruker 360 MHz proton probe. We report here an alternative modification utilizing fiber optics illumination. Coupling to the laser was accomplished with a Newport Corporation (Fountain Valley, Calif.) model F-LFI-CO-25 ($850.00) fiber optics coupler laser head mount (universally mounts to any laser bezel mount) with 25 ft of 200 p fiber in place of the standard 10 ft length and armored handpiece. The free output end was stripped free of cladding (l-2 cm length) and inserted into a 2 mm stem coaxial insert (Cat. No. 5203, Wilmad Glass Company, Inc., Buena, N.J.) which fits snuggly into a 5 mm sample tube (Fig. 1). Maximal illumination was observed when the fiber tip was positioned in the center of the rf coil region. The laser light probe was introduced by triggering an inexpensive solenoid mounted to the coupling head slide shutter under computer control. Light coupling was fine tuned with x, y, and z adjustments on the coupling head. For maximal efficiency the fiber must be cut perfectly flat and well polished. Coupling efficiencies of up to 90% were typically measured. In a nonspinning sample of 1 mA4 tyrosine (pH 7.0) and 0.4 r&4 ‘?N-carboxyethyllumiflavin, the tyrosyl 3,5 protons were enhanced (light intensity-dark intensity)/ (dark intensity) 1Zfold at 500 MHz on an NTC spectrometer with a Spectra Physics 165-9 argon ion laser operating at 7.0 W (all lines). Figure 2 displays the light-dark