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: A significant proliferative response to serum collected after dilevalol intake was observed in the case of the patient compared with the proliferative response to the serum collected before the drug intake .
### Output Text:
A significant proliferative response to serum collected after <span class="chemical">dilevalol</span> intake was observed in the case of the patient compared with the proliferative response to the serum collected before the drug intake . 