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: This last finding may be related to time of testing and / or endogenous serum binding of rifampin which could have inhibited mitogen activity for the lymphocyte .
### Output Text:
This last finding may be related to time of testing and / or endogenous serum binding of <span class="chemical">rifampin</span> which could have inhibited mitogen activity for the lymphocyte . 