Cyanine dyes and their analogs are commonly used as acceptors in commercially available antibodytandem fluorophore conjugates. However, it has been confirmed that the dyes have a tendency to bind monocytes or macrophages nonspecifically, which limits the ability to do multicolor flow cytometric analysis for lower density antigens. We are introducing TrueStain Monocyte Blocker™ buffer which eliminates this nonspecific binding. This reagent blocks nonspecific binding of Cyanine dyes (PE/Cy7, PE/Cy5, PercpCy5.5, APC/Cy7, APC/Fire™ 750, PE/Dazzle™ 594) to monocytes and macrophages. It does not affect binding of e.g., CD64, CD14 and other tested antibodies. Multiple Cyanine dyes can be blocked at once and stability data shows no decrease in signal intensity compared to the conjugate alone. In addition the reagent has no impact on cell viability. Mock sorting experiments were performed with PBMC to determine if the TrueStain Monocyte Blocker™buffer shows inhibitory or enhanced function on inflammatory cytokine/chemokine production. Our data indicates minimal to no effect above conjugated antibody for IL6, IL8, IL10 and MCP1 production. We also tested TrueStain Monocyte Blocker™ buffer for its ability to affect PBMC proliferation in response to antiCD3/ antiCD28 stimulation. We observed minimal to no effect on proliferation by measuring BrdU incorporation.