Prop 38 would provide nearly $8.5-billion for immunology research.
Supporters say the measure could help advance cures for cancer, heart disease, and Alzheimer's.
Opponents point out that the proposition designates half of the funds to just one unnamed research institute.
That institute appears to be the California Institute for Immunology and Immunotherapy based in the UCLA research park.
The primary backer of Prop. 38 is billionaire Gary K. Michelson, who co-founded that institute.
Opponents also question whether it's fair to commit so much money to just one institute when California has so many world-class medical researchers.
In response to these concerns, the Yes on 38 campaign has said that the prop is written to ensure that the funds go to a high-quality institution.