The microbial oxidation of ebastine to carebastine was investigated. Among the 15 micro-organisms examined, only the Cunninghamella strains showed the desired biotransformation. Cunninghamella blakesleeana oxidised the substrate within 7 days, via the intermediates alcohol and aldehyde, mainly to carebastine, the corresponding carboxylic acid. Optimisation of the culture conditions increased the yield from initially 10% up to a reproducible 40%. For the synthesis of carebastine a substrate concentration of 200 mg/l, a starting pH of 5.0 and the addition of 1% poly(vinyl alcohol) is favourable. The results achieved in experiments with shaking flasks are transferable to the fermentation scale and yielded 270 mg carebastine in a 3-1 fermentation of 600 mg ebastine. The progress of the reaction was detected by TLC and HPLC, the products were identified by mass spectrometry and NMR.