Infant formula samples were analysed to determine their free L-carnitine content by using flow injection (FI) with an incorporated immobilised carnitine acetyltransferase bioreactor. The methodology was based on the spectrophotometric determination through its reaction with carnitine acetyltransferase coupled with acetyl coenzyme A (acetyl-CoA) and dithiobenzoate. The merging zones technique was used to minimise acetyl CoA consumption. Linearity was observed over the concentration range 10-80 mg l-1 with L-carnitine as standard (r = 0.9998) and the rate of analysis was 50 h-1 infant formula samples. The enzymic FI method afforded a low RSD (2.2%). Comparisons were made with other methods of known accuracy. The enzymic reactors were stable for 3 months when used daily at the optimum pH.