Saracens and former South Africa lock Alistair Hargreaves has announced his retirement as a result of several concussion injuries.
The 30-year-old, who won four Caps for the Springboks, moved to Saracens from Super Rugby outfit Sharks in 2012.
In his first season as captain, he led the club to the 2014-15 Premiership title.
"It's been a very tough decision to make," Hargreaves told Saracens' website. "I've suffered a number of concussions in the last two seasons and after seeking medical advice I've come to the conclusion that the risk is outweighing the reward.
"I have a young family, I have to make decisions that are right by them and although this was a hard decision for me personally it's one that I feel is the right one to make.
"The club have been amazing ever since I walked through the doors, but especially over the last 12 months when I've been suffering from injuries and concussion.
"They have been a constant source of support in a really emotional and difficult time."
The Rugby Football Union announced enhancements to the way concussion will be managed at professional club level in England last month.
Part of the new initiative will give medical staff access to real-time pitch-side video on a trial basis through the "myplayXplay" system in a bid to boost the recognition of serious head injuries.
In addition, the allotted time for a head injury assessment to be undertaken on a player has been extended from 10 to 13 minutes in order to include a King-Devick test, which scans eye movement, though it is only being used for research this season.
Furthermore, a mandatory concussion education programme is again being delivered to players, coaches, support staff and referees in the country's top two divisions.