Hi Sid,
Although it is not written to the lead record, the lead's
user agent string is recorded as part of Visit Web Page and Fill Out Form activities (and maybe others; feel free to add to that list).
You can see this by double-clicking on a Visit Web Page entry in the Activity Log, and viewing the detail popover. See the screenshot below for an example.
![0EM50000000Sk4M.jpg 0EM50000000Sk4M.jpg](/t5/image/serverpage/image-id/11352i9409908337E27379/image-size/large?v=v2&px=999)
This user agent string does identify the lead's browser, but it is pretty clunky. There's a huge variety of user agent strings, and there's a lot of information in there. Pasting the whole thing into
http://www.useragentstring.com/ can yield some insight, though.