The marketing services group will initially pay £2.5m for the agency, with additional payments of up to £3.3m dependent on performance during the period to 31 December 2016.
iS is an agency specialising in medical communication services to UK, European and global healthcare clients and will add to Cello’s capabilities in evidence-based communications.
The Farnham-based business was established by Isaac Batley and Steve Smith in 2003 and employs around 50 people.
It had a turnover of £6.6m for the year to 31 March 2014 and a gross profit of £3.9m.
Financial PR agency Buchanan is supporting Cello's communications around the deal.