Florida-based PositiveID Corp. on Thursday announced an offer to buy Digital Angel, the South St. Paul manufacturer of animal tags for tracking livestock.

In a statement, Digital Angel called the offer "unsolicited and non-binding." The company said its board would review the offer at its next regularly scheduled meeting later this month.

The offer from PositiveID is a stock-for-stock transaction valued at 60 cents a share. The company said that represents a 60 percent premium over the average trading price for the last 20 days. Digital Angel shares rose 10 cents Thursday and were trading near 50 cents a share. Shares of PositiveID fell a nickel to close at 77 cents.

In a letter to the board of Digital Angel, PositiveID said it has been rebuffed by Digital Angel several times when it tried to address a business combination.

PositiveID said it believed a combination would build on synergies between the two companies and expand product lines. A merger would also "offer significant value" to both companies shareholders, the letter said.

PositiveID makes health care and information management products using RFID technology.