Enzyme-linked　immunosorbent　assay　(ELISA)　methods　based　on　natural　enzyme-labeled　probes　have　been　applied　in　the　immunoassays,　but　most　have　some　inevitable　limitations　(e.g.　harsh　preparation,　purification　and　storage)　and　are　unsuitable　for　routine　use.　Herein　we　synthesized　a　new　class　of　irregular-shaped　platinum　nanoparticles　(ISPtNP)　with　a　mean　length　of　7.0nm　and　a　narrowing　width　from　2.0　to　5.0nm　along　the　longitudinal　axes,　which　were　utilized　as　peroxidase-like　mimics　for　the　development　of　colorimetric　immunoassays.　Compared　with　bioactive　horseradish　peroxidase　(HRP),　the　synthesized　ISPtNP　exhibited　a　low　Km　value　(~0.12mM)　and　a　high　Kcat　value　(~2.27×104s-1)　for　3,3＇,5,5＇-tetramethylbenzidine　(TMB)　with　strong　thermal　stability　and　pH　tolerance.　The　catalytic　mechanism　of　the　ISPtNP　toward　TMB/H2O2　was　for　the　first　time　discussed　and　deliberated　in　this　work.　Based　on　a　sandwich-type　assay　format,　two　types　of　colorimetric　immunoassay　protocols　were　designed　and　developed　for　the　detection　of　rabbit　IgG　(RIgG,　as　a　model)　by　using　the　synthesized　ISPtNP　and　conventional　HRP　as　the　labeling　of　detection　antibodies,　respectively.　Similar　detection　limits　(LODs)　of　2.5ngmL-1　vs.　1.0ngmL-1　were　obtained　toward　RIgG　with　the　ISPtNP　labeling　compared　to　HRP　format.　Intra-　and　inter-assay　coefficients　of　variation　were　less　than　13%.　Importantly,　the　ISPtNP-based　assay　system　could　be　suitable　for　use　in　a　mass　production　of　miniaturized　lab-on-a-chip　devices　and　open　new　opportunities　for　protein　diagnostics　and　biosecurity.　©　2013　Elsevier　B.V.