The 29,000-Mr Actinomadura R39 beta-lactamase exhibited a remarkably low electrophoretic mobility on SDS/PAGE, yielding an Mr value almost twice that computed from the corresponding gene sequence. We showed that chemical modification of the carboxylic groups of glutamic acid and aspartic acid residues restored a normal electrophoretic mobility and that the anomalous behaviour of that protein on SDS/PAGE was due to its very large negative charge at neutral pH. We also compared the behaviour of the same enzyme on gel filtration in the presence of SDS with those of other class A beta-lactamases (Mr approx. 30,000). These experiments suggested that the very low electrophoretic mobility of the Actinomadura R39 beta-lactamase upon SDS/PAGE was more probably due to a low degree of SDS binding rather than to an unusual shape of the SDS-protein complex.