Give a stream of consciousness and then the final answer. Gram-negative bacteria are a group of bacteria that do not retain the crystal violet stain used in the gram-staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane.

Answer this question: the cell wall of gram-negative bacterial cells is external to the?
To answer the above question, the relevant sentence is: They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane.
So, the final answer is an inner cytoplasmic cell membrane.