Barbiturates are a source of interference in the estimation of paracetamol by both UV spectrophotometry and GLC. The interference by barbiturates on GLC can be avoided by benzoylation (in aqueous solution) of the paracetamol. This O-benzoyl derivative can be N-silylated to give an additional identification parameter. The method, which requires 1 ml of sample, will quantitatively and qualitatively estimate "free" paracetamol in plasma and in post mortem blood in the range of 2-40 .mu.g of drug per ml of sample.