Serine peptidases (SP) are peptidases with a uniquely activated serine residue in the substrate-binding site. SP can be classified into clans with distinct evolutionary histories and each clan further subdivided into families. We analyzed 79 proteins representing the S1A subfamily of human SP, obtained from different databases. Multiple alignment identified 87 highly conserved amino acid residues. In most cases of substitution, a residue of similar character was inserted, implying that the overall character of the local region was conserved. We also identified several conserved protein motifs. 7-13 cysteine positions, potentially forming disulfide bridges, were also found to be conserved. Most members are secreted as inactive (pro) forms with a trypsin-like cleavage site for activation. Substrate specificity was predicted to be trypsin-like for most members, with few chymotrypsin-like proteins. Phylogenetic analysis enabled us to classify members of the S1A subfamily into structurally related groups; this might also help to functionally sort members of this subfamily and give an idea about their possible functions.