Proteins naturally expressed in eukaryotic organisms often require host chaperones, binding partners, and posttranslational modifications for correct folding. Ideally the heterologous expression system chosen should be as similar to the natural host as possible. For example, mammalian proteins should be expressed in mammalian expression systems. However this does not guarantee a protein will be expressed in a sufficient high yield for structural or biochemical studies or antibody generation. Often a screening process is undertaken in which many variants including truncations, point mutations, investigation of orthologues, fusion to peptide or protein tags at the N- or C-terminus, the co-expression of binding partners, and even culture conditions are varied to identify the optimal expression conditions. This requires multi-parallel expression screening in mammalian cells similar to that already described for E. coli expression. Here we describe in detail a multi-parallel method to express proteins in mammalian suspension cells by transient transfection in 24-well blocks.