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: H1 - and H2 - histamine antagonists or placebo were given before dosing with d - tubocurarine in a randomized double - blind fashion to four groups : group 1 - - placebo ; group 2 - - cimetidine , 4 mg / kg , plus placebo ; group 3 - - chlorpheniramine , 0 . 1 mg / kg , plus placebo ; and group 4 - - cimetidine plus chlorpheniramine .
### Output Text:
H1 - and H2 - <span class="chemical">histamine</span> antagonists or placebo were given before dosing with <span class="chemical">d - tubocurarine</span> in a randomized double - blind fashion to four groups : group 1 - - placebo ; group 2 - - <span class="chemical">cimetidine</span> , 4 mg / kg , plus placebo ; group 3 - - <span class="chemical">chlorpheniramine</span> , 0 . 1 mg / kg , plus placebo ; and group 4 - - <span class="chemical">cimetidine</span> plus <span class="chemical">chlorpheniramine</span> . 