SEPT. 4–CONCORD, Calif.- Shares of Cerus Corp. began to sank today after the biotechnology company and corporate partner Baxter International Inc. said they stopped a human experiment designed to make blood transfusions safer.

The companies said two patient volunteers in the test showed signs of resistance to the experimental blood sterilization chemical jointly developed by the two companies.

The two patients didn’t experience any adverse physical reactions, but the companies still halted their late-stage experiment as a “precautionary measure.”

Concord-based Cerus stock closed down $2.42 a share, or 31 percent, to $5.29 in trading on the Nasdaq Stock Market. Trading in Deerfield, Ill.-based Baxter closed unchanged at $28.49 in trading on the New York Stock Exchange.

The companies said they are evaluating the test results to determine whether to continue their pursuit of “pathogen inactivation” technology in red blood cells.

The companies are developing a chemical compound dubbed S-303, which when added to a donated bag of blood is supposed to destroy genetic material of viruses or bacteria lurking inside