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: In long - term infusion studies , flestolol was well tolerated at the effective beta - blocking dose ( 5 micrograms / kg / min ) for up to seven days .
### Output Text:
In long - term infusion studies , <span class="chemical">flestolol</span> was well tolerated at the effective beta - blocking dose ( 5 micrograms / kg / min ) for up to seven days . 