The polymorphism of the serum cholinesterase CHE 1 was determined using an automated micro-method. The procedure developed represents an improvement of the manual method and enabled us to study more than 2400 samples obtained in 15 different geographical areas in France. Four alleles were detected: CHE1* U, A, S and F. An exceptionally high frequency of an atypical variant (CHE1*A) was observed in the central part of France (Cevennes, Limousin and Dauphiné). The populations belonging to these areas are at particular risk in case of anaesthesia.