Hooker Marx has a tibia fracture but is expected to be fit for the Springboks' second fixture against Australia in Perth on August 17. Loose forward Du Toit should be ready for their opener a week earlier against the Wallabies in Brisbane.
The squad will be led by captain Siya Kolisi. Returning lock Ruan Nortje is the only player included in the group who missed the drawn two-Test series with Ireland and victory over Portugal earlier this month.
He comes in for Franco Mostert, who heads an injury list that includes second rowers Jean Kleyn and Lood de Jager, prop Steven Kitshoff, scrumhalves Faf de Klerk and Jaden Hendrikse, centre Canan Moodie and fullback Damian Willemse.
They join suspended No.8 Jasper Wiese and centre Andre Esterhuizen on the sidelines, the latter waiting for the verdict of his disciplinary hearing after being sent off in the 64-21 win over Portugal on Saturday.
All were part of the squad that lifted the Rugby World Cup in France last October, but their absence gives coach Rassie Erasmus the chance to cast his selection net wider.
"This is a quality group of players, which includes a mixture of youth and experience, and we believe this is the best group of players we could select," Erasmus said in a media release from South African Rugby.
"We rested some players before the match against Portugal to recharge for this competition and we are excited to see the energy they'll bring back to the squad.
"We've also included a few younger players who made a strong statement in the Tests we played so far this season."
Erasmus added they will review the squad ahead of two home matches against New Zealand later in the competition and the home and away fixtures against Argentina.
South Africa squad:
Forwards: Ben-Jason Dixon, Pieter-Steph du Toit, Thomas du Toit, Eben Etzebeth, Johan Grobbelaar, Vincent Koch, Siya Kolisi (capt), Elrigh Louw, Frans Malherbe, Malcolm Marx, Bongi Mbonambi, Salmaan Moerat, Ox Nche, Ruan Nortje, Kwagga Smith, RG Snyman, Gerhard Steenekamp, Marco van Staden, Jan-Hendrik Wessels
Backs: Lukhanyo Am, Kurt-Lee Arendse, Damian de Allende, Aphelele Fassi, Sacha Feinberg-Mngomezulu, Cheslin Kolbe, Jesse Kriel, Willie le Roux, Makazole Mapimpi, Manie Libbok, Handre Pollard, Cobus Reinach, Morne van den Berg, Grant Williams.
