We employ a massively parallel reporter assay (MPRA) to measure the ex vivo activities of hundreds of K562 and HepG2 enhancers with known transcription factor motif instances. For seven selected motifs that correspond to known or predicted activators and repressors in the two cell types, we make directed modifications of the bases corresponding to these motifs and observe the changes in enhancer activity. Reporter mRNA-seq from HepG2 and K562 cells transfected with a ~55,000-plex MPRA plasmid pool containing 5,418 mutated human enhancer sequences, each linked to 10 distinct 10-nt tags. The reporter mRNA tags facilitate quantitation of their abundances. The same tags were also sequenced from the transfected MPRA plasmid pool to facilitate normalization to plasmid copy numbers.