Packed-column supercritical fluid chromatography (pSFC) using ultra-violet (UV) and atmospheric pressure chemical ionisation (APCI) mass spectrometry (MS) provides a versatile method for the identification and quantification of beta-agonists. We have achieved good separation of clenbuterol, salbutamol, terbutaline and fenoterol with good resolution and reasonable retention times using a high concentration of methanol modifier in the supercritical CO2, together with small amounts of both acidic (trifluoroacetic acid, TFAA) and basic (triethylamine, TEA, or diethylamine, DEA) additives. APCI-MS gave unambiguous identification of the 4 analytes, and increasing cone voltage provided informative fragmentation patterns. The pSFC-MS technique was shown to be linear (R2 > or = 0.996) over the concentration range 1-50 micrograms ml-1. Single ion monitoring (SIM) gave detection limits (on-column) of 2.5 ng (clenbuterol), 0.83 ng (terbutaline), 7.6 ng (salbutamol) and 2.7 ng (fenoterol). The pSFC-MS system was shown to be reproducible within a day, between days, and between restrictors. Analysis of milk samples 'spiked' with beta-agonists showed that the matrix caused no interference, with detection limits of approximately 500 micrograms l-1 of beta-agonists. More dilute solutions could be analysed by pre-concentration before the SFC stage.