Ionizable lipids are established nucleic acid delivery vehicles capable of delivering mRNA to induce protein expression in target cells. These lipids deliver their mRNA cargo into the cytosol, where it can be directly translated into proteins. As a result, lipid-based mRNA delivery does not carry the risks of mutagenesis that limit the clinical use of both viral vectors and DNA-containing delivery systems. Yet, current lipid-based delivery systems suffer from low transfection efficiency, especially for cell types such as primary cells and immune cells, which are notoriously resistant to transfection due in part to their immune response to foreign nucleic acids and selective uptake of molecules by specialized endocytic machinery. To address this, we designed a library of 4,200 ionizable lipids of which a panel of 84 potential candidate vehicles were synthesized and screened in human peripheral mononuclear cells (PBMCs) and other difficult-to-transfect immune cells including THP-1 monocytes. Human PBMCs were transfected with green fluorescent protein (GFP)-encoding mRNA formulated into complexes with the candidate lipids. 43 of these lipids possess a spermine-derived backbone, a structural characteristic of the ToRNAdo™ Nucleic-Acid Delivery System which has effectively delivered mRNA to a variety of cell types both ex vivo and in vivo. We also tested these lipids on PBMCs that had been exposed to T-cell activators IL-2 and anti-CD3/CD28 antibodies for 72 hrs to mimic innate immune activity in a pathogenic microenvironment. 1.33 ng/uL of GFP mRNA was delivered per 100,000 PBMCs at a lipid:mRNA wt/wt ratio of 3:1. High content cellular imaging and fluorescent microscopy showed peak GFP fluorescence intensity 24-36 hrs after transfection. Flow cytometry was performed 24 hrs after transfection to quantify GFP expression and viability in transfected cells. Of the lipids tested, those containing spermine headgroups and hexyldecanoate ester tails yielded high levels of GFP expression in CD3+ cells with a four- to eleven-fold increase in GFP positive cells relative to Lipofectamine™ 3000. The same lipids transfected THP-1 monocytes at greater than 90% efficiency. Our data collectively indicates that spermine-derived polyvalent ionizable lipids target transfection of mRNA to primary human immune cells, particularly T cells and monocytes. These novel lipids may prove useful in the development of mRNA therapeutics based on the transfection of immune cells.