Abstract A simple, rapid, and sensitive chemical method is described for determining lasalocid sodium in finished poultry feeds and premixes. The drug is extracted with ethyl acetate directly from premixes and after a brief treatment with pH 4.7 buffer from finished feeds. Lasalocid sodium concentration is measured spectrofluorometrically and a correction for nonspecific fluorescence is determined after formation of a lasalocid- boric acid complex. Replicate assays of typical poultry mashes and pellets containing about 0.0075% lasalocid sodium yielded coefficients of variation of 2.8 to 5.0%. Average recovery of lasalocid sodium added in the laboratory at 0.0037, 0.0075, and 0.0112% was 90% with a pooled standard deviation of ±1.86%.