In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Skin tests and in vitro lymphocyte stimulation revealed that the patient was sensitized to cephalothin and also to ampicillin .
### Output Text:
Skin tests and in vitro lymphocyte stimulation revealed that the patient was sensitized to <span class="chemical">cephalothin</span> and also to <span class="chemical">ampicillin</span> . 