The sequences of class A beta-lactamases were compared. Four main groups of enzymes were distinguished: those from the gram-negative organisms and bacilli and two distinct groups of Streptomyces spp. The Staphylococcus aureus PC1 enzyme, although somewhat closer to the enzyme from the Bacillus group, did not belong to any of the groups of beta-lactamases. The similarities between the secondary structure elements of these enzymes and those of the class C beta-lactamases and of the Streptomyces sp. strain R61 DD-peptidase were also analyzed and tentatively extended to the class D beta-lactamases. A unified nomenclature of secondary structure elements is proposed for all the penicillin-recognizing enzymes.