Slovak striker Milos Lacny has arrived in Glasgow for a second trial with Celtic - a year and a half since his last visit to the club.
And the 23-year-old is joined on trial by South Africa forward Katlego Mphela.
Lacny impressed on his last trial stint in December 2009 but decided instead to sign for Sparta Prague.
The Slovakia Under-21 cap had been with Slovak top-flight club Ruzomberok before switching to the side who finished second in the Czech Republic.
South Africa international Mphela, who was a target for Rangers last summer, has spent the last three years in his homeland's Premier League with Mamelodi Sundowns.
The 26-year-old had joined the club from Supersport United but has previous experience in Europe.
After departing Jomo Cosmos, he had a two-year spell with Strasbourg during which he was sent out on loan to Stade Reims.