The solid-phase synthesis of a 738,192 member pyrrolidine bis-cyclic guanidine chemical library with four positions of diversity (R 1–R 4) is reported. Screening this library yielded eight compound mixtures with bactericidal activity against methicillin-resistant Staphylococcus aureus. Thirty-six individual compounds from these mixtures exhibited potent bactericidal activity against important human pathogens. The rapid rise in antibiotic-resistant Gram-positive bacterial infections prompted us to explore the development of novel strategies for synthesis of large chemical libraries amenable to high-throughput screening for antimicrobial activities. Here we report the solid-phase synthesis of a 738,192 member pyrrolidine bis-cyclic guanidine chemical library with 26 different amino acids at three positions of diversity and 42 carboxylic acids at the fourth position. This synthetic combinatorial library was developed for positional scanning and screened for bacteriostatic and bactericidal activities against the important human pathogen methicillin-resistant Staphylococcus aureus (MRSA). The eight compound mixtures exhibiting bactericidal activity (10 μg/mL) against MRSA were used to direct the synthesis of 36 individual compounds that were then screened for activity against MRSA, vancomycin-resistant Enterococcus faecalis (VRE), and two Gram-negative bacterial species. At least 20 individual compounds were bactericidal for MRSA at ⩽2.5 μg/mL, with a subset of these compounds showing bactericidal activities (⩽10 μg/mL) against the other species tested. This approach demonstrates the capability to synthesize and screen a complex library to yield promising antimicrobials that address a critical need for novel infectious disease therapeutics.