Black Caviar's half-brother is fighting for his life in a Victorian equine clinic.

It is believed the colt, bought for a record $5 million at the Easter Yearling Sale in Sydney, was bitten by a white-tailed spider two weeks ago which caused swelling in a front leg.

The next 48 hours are vital to his survival.

Flemington equine vet Tom Brennan explained that in the process of treating the swelling the colt, known as Jimmy, had a reaction to antibiotics which has led to an onset of the hoof disease laminitis in his two front feet.

He said the prospects of a horse making it to the racetrack after contracting laminitis were slim.

"His racing prospects are guarded," Dr Brennan said.

"We're hopeful, if he has no further complications, there is still a slim chance we may be able to get him to the racetrack but, at the moment, we're focused on trying to save him and his breeding career."

He said the valuable colt was receiving powerful painkillers to keep him comfortable and was wearing special boots on his two front feet in an effort to stabilise him.

"His life is on the line, he's a 50-50 prospect and we hold grave concerns for him," Dr Brennan said.

Simon Marshall, a spokesman for the BC3 Thoroughbreds syndicate which purchased the youngster, said everyone involved with the horse was devastated.

"We invest our lives in horses and to have him as a member of our team was a dream," Marshall said.

"We were really looking forward to a bright future."

Black Caviar was unbeaten in her career, winning 25 consecutive races including 15 Group Ones and accumulating almost $8 million in prizemoney.