A private company is on the verge of launching another cargo ship to the International Space Station.

California-based SpaceX will attempt to send a Dragon capsule to the orbiting lab on Sunday night. Liftoff is scheduled for 8.35pm EDT (1235 AEDT Monday).

Forecasters put the odds of acceptable weather at 60 per cent. Thick clouds and rain are the main concerns.

A Dragon cargo ship successfully docked to the space station last May, but that was considered a test flight.

The coming mission is the first under a $1.6 billion contract with NASA that calls for a dozen resupply flights by SpaceX, essential in the post-shuttle era.

The Dragon will remain docked to the space station for most of October. Astronauts will fill the capsule with blood and urine samples, other experiments and old equipment, for its return to Earth.

The nearly 500 tubes of blood and syringes of urine have been stashed in space station freezers since the last space shuttle flight, by Atlantis, in July 2011.

The decommissioned Atlantis, and sister ships Discovery and Endeavour, are now museum relics.

NASA nutritionist Scott Smith said the blood and urine samples - part of medical studies - will be the first to be returned since Atlantis' final voyage more than a year ago.

"This is the first real return vehicle for this type of sample," Scott told reporters on Saturday.

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Private space station delivery to launch