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WebBrowser1.OleObject.document.forms.item('Formname').elements.item('Radiobutton').value('1');                    

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<form NAME="Kramform">
<input TYPE="radio" NAME="Kram">Kram 1<br>
<input TYPE="radio" NAME="Kram">Kram 2
</form>                    

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WebBrowser1.OleObject.document.forms.Kramform.Kram[1].checked:=checked;                    

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WebBrowser1.OleObject.Document.Forms.item('Formname').Elements.item('boxname').checked:=true;                    

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document.mysignup._sexe[0].checked = true;                    

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document.mysignup._sexe[1].checked = true;                    

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procedure CheckHTMLRadioButton(Doc: IHTMLDocument2; Formname, RBName, RBValue: string);
var
  IHTMLForm: IHTMLFormElement;
  IHTMLInput: IHTMLInputElement;
  IHTMLElem: IHTMLElement;
  IElements: IHTMLElementCollection;
  i,j: integer;

begin
for i := 0 to Doc.forms.length-1 do
  begin
  IHTMLForm := Doc.forms.Item(i, i) as IHTMLFormElement;
  if IHTMLForm.name = Formname then
    begin
    IHTMLForm.QueryInterface(IID_IHTMLElement, IHTMLElem);
    IElements := IHTMLElem.all as IHTMLElementCollection;
    for j := 0 to IElements.length-1 do
      begin
      IHTMLElem := IElements.Item(j,j) as IHTMLElement;
      if (IHTMLElem.tagName = 'INPUT') then
        begin
        IHTMLInput := IHTMLElem as IHTMLInputElement;
        if (IHTMLInput.name=RBName) and (IHTMLInput.value = RBValue) then
          IHTMLInput.checked := true;
        end;
      end;
    end;
  end;
end;

procedure CheckRadioButton(Browser: TWebBrowser; Formname, RBName, RBNumber: string);
var
  win: IHTMLWindow2;

begin
win := (Browser.Document as IHTMLDocument2).parentWindow;
win.execScript('document.'+Formname+'.'+RBName+'['+RBNumber+'].checked=''checked''','javascript');
end;

procedure TForm1.Button3Click(Sender: TObject);
begin
CheckRadioButton(WebBrowser1,'mysignup','_sexe','1'); // oder '0' 
end;

procedure TForm1.Button4Click(Sender: TObject);
begin
CheckHTMLRadioButton(Webbrowser1.Document as IHTMLDocument2, 'mysignup','_sexe','1'); // oder '2'
end;