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 contrast , maximal platelet aggregation increased in the acetaminophen group on the third postoperative day as compared with the pretreatment platelet aggregation results ( P < . 05 ) .
### Output Text:
In contrast , maximal platelet aggregation increased in the <span class="chemical">acetaminophen</span> group on the third postoperative day as compared with the pretreatment platelet aggregation results ( P < . 05 ) . 