Complement is a key component of the immune system, but can contribute to inflammatory diseases. The substrate specificity of C1s protease has been successfully investigated using a combinatorial approach, while a positional scanning method failed. The lack of success of the latter approach is possibly due to cooperativity in the active site, which could confound such analyses. With a panel of peptides devised using factorial design, we show pronounced cooperativity between the S4 and S1' subsites in the active site of the enzyme, and weaker cooperativity between the S1' and S3' subsites. The use of factorial design has promise as a methodology for determining cooperativity in protease active sites.