Serotyping was carried out on 80 coded strains, distribute to all laboratories taking part in the WHO L. monocytogenes multicenter subtyping study. All six laboratories used the method recommended by their coordinator. All 80 strains were typeable. There was complete agreement between the six laboratories on 49 (61.3%) strains (21 serovar 1/2a and 28 serovar 4b strains) which in turn were identical to the expected serovars, known only after decoding. The intralaboratory reproducibility carried out on 11 duplicate strains, ranged from 82 to 100%, with a medium value of 91%. Reproducibility of serotyping L. monocytogenes strains according to serovar varied from 33.3 to 100% for serotypes 3b and 1/2a, respectively, with serovar 4b (x) being incorrectly identified in all six laboratories. Serotyping of L. monocytogenes is easy and simple and is a useful prerequisite for other finer and more discriminatory typing methods. Problems may however, be encountered mainly with the flagellar antigenic factors. There is a need, therefore, for preparing antisera of good quality from which efficient antigenic factors can be obtained.