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: Intravenous or intratracheal inoculation of 2 . 0 to 2 . 5 mg of vincristine sulfate was followed by leukopenia in 4 to 5 days .
### Output Text:
Intravenous or intratracheal inoculation of 2 . 0 to 2 . 5 mg of <span class="chemical">vincristine sulfate</span> was followed by leukopenia in 4 to 5 days . 