in the spectrum of lasalocid, which is assigned to ( -+ Na)Na+. With monensin spotted on a TLC plate, the sensitivity of detection was established as well below 0.1 µg.A major concern was the possible interference related to the components in the TLC adsorbent and the masking tape. However, no background interference was observed in the mass range of interest based on blank tests of both fluorescent and nonfluorescent TLC plates and direct tests of the masking tape. Also, contamination of the ion source by the TLC adsorbent, i.e., silica gel, was not observed. However, after the depletion of the FAB matrix liquid, the flake of adsorbent from plastic-backed TLC sheets appeared to remain intact while that from glass TLC plates tended to disintegrate. For this reason we favor the Use of plastic-backed TLC sheets.In conclusion, we have demonstrated a simple and rapid method for directly obtaining FAB spectra of TLC fractions which circumvents the problems associated with the previously cited methods. ACKNOWLEDGMENTWe wish to thank D. B. Borders and . T. Lee of Lederle Laboratories, American Cyanamid Company, for providing the coccidiostats and the TLC separation procedure.LITERATURE CITED(1) Deverse, F.