Tylosin is a widely used antibiotic for the treatment of infections in swine. Tylosin consists of a mixture of Tylosin A, Tylosin B, Tylosin C and Tylosin D. All components contribute to the potency of tylosin but Tylosin A is by far the major component (usually about 90% and not less than 80%). A fast, robust and easily performed HPLC method has been developed for determination of Tylosin A in the presence of tylosin residues; Tylosin B, Tylosin C and Tylosin D in manure containing incubation media. The separation was performed using a YMC-Pack ODS-AQ column (250 x 4.6 mm i.d., 5 microns particle size) operated at 35 degrees C. The mobile phase consisted of 2.25% (w/v) sodium perchlorate pH 2.5-acetonitrile (60:40 v/v). Detection was performed by measuring the UV absorption at a wavelength of 290 nm. Calibration curves of tylosin made in the incubation medium containing 6.4% manure were linear in the range from 0.375 to 128.0 mg/l (R2 = 0.999). The limit of quantitation (at the RSD 20% level) for Tylosin A was found to be 0.4 mg/l in incubation media containing 6.4% manure. The recovery of Tylosin A was in the range from 100% to 108% depending on the concentration of manure. The reproducibility was good as the relative standard deviation (n = 4) in each matrix tested was in the range from 0.7 to 1.9 at the 25 mg/l level. The stability of Tylosin A was studied under methanogenic conditions and the half-life was found to be less than two days. Studies under aerobic conditions showed that the degradation rate was found to increase with increasing concentrations of manure particles in the incubation medium. It is, however, not clear whether the decrease in the concentration of Tylosin A is caused by sorption, abiotic or biotic chemical degradation. The major degradation product of Tylosin A in methanogenic as well as aerobic incubation media has a UV-spectrum and a retention time corresponding to Tylosin B. Furthermore, Tylosin D is believed to be a minor degradation product.