The combination of baculovirus-mediated gene delivery (BacMam) with LanthaScreen® Cellular Assay technology enables a platform for the analysis of specific posttranslational modifications of histones. BacMam provides a convenient genetic delivery tool for a GFP-Histone H3 fusion protein in the cell line of interest. This kit describes an HTS-compatible cellular immunoassay measuring phosphorylation of GFP-Histone H3 at Ser10.