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: Positive MIF test for ampicillin showed sensitization of the patient ' s lymphocytes to ampicillin .
### Output Text:
Positive MIF test for <span class="chemical">ampicillin</span> showed sensitization of the patient ' s lymphocytes to <span class="chemical">ampicillin</span> . 