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: Superoxide and hydrogen peroxide production by ALF neutrophils stimulated with zymosan opsonized with ALF serum was significantly reduced compared with the control subjects ( P < 0 . 01 ) .
### Output Text:
<span class="chemical">Superoxide</span> and <span class="chemical">hydrogen peroxide</span> production by ALF neutrophils stimulated with zymosan opsonized with ALF serum was significantly reduced compared with the control subjects ( P < 0 . 01 ) . 