UnitedHealthcare, the giant nationwide insurer, is making a major move in South Florida by announcing Tuesday it has agreed to purchase two Miami-Dade based Medicare and Medicaid insurance plans that have more than 100,000 members and eight clinics.

Terms of the two deals were not announced.

Preferred Care Partners has 55,000 Medicare members in its health maintenance organization in South and Central Florida and another 5,000 members in its Medicaid HMO. It has six clinics in Miami-Dade.

Medica Healthcare Plans has 35,000 Medicare HMO members and 7,200 Medicaid beneficiaries in Miami-Dade and Broward. It has two clinics, in Coral Gables and Hialeah.

“This is certainly a bold move by United,” said Joseph Caruncho, founder and chief executive officer of Preferred Care. “It shows their commitment to become involved in South Florida and immediately become a major player.”

Caruncho said he and most of the Preferred Care team will stay after the sale is completed. He called the new relationship a partnership in which UHC will benefit by “our relationship with the community and the seniors.”

Medica did not immediately respond to a request for comment.

UHC made an earlier foray into the Miami-Dade market that proved disastrous in an area where Hispanic seniors expect to sip cafecitos and have an area for socializing while waiting for treatment. In 1994, UHC spent $500 million for another HMO, CAC Ramsey HMO and its 18 clinics. The Minneapolis-based UHC “didn’t understand the culture,” said former CAC Ramsey executive Miguel “Mike” Fernandez in a 2007 interview. They stopped the cafecitos and “wasted their time and money.”

Within five years, UHC sold the 18 clinics, and Fernandez eventually picked them up for less than $100 million, returning them to the previous model and building them into a CarePlus network that was worth more than $450 million when sold to Humana in 2004.

UHC spokesman Matt Burns said the company “today has a great deal more experience with and commitment to local markets and seniors, combined with substantially more data, analytics and resources.”

He said the company’s goal is to serve its growing number of customers well “and maintain the trust they place in us.”

The purchases are expected to close later this year, following regulatory approvals.

Caruncho said Preferred Care will retain its name and network for the rest of 2012. What happens after that remains to be seen. He said the company, which includes a large minority stake by investment firm Ferrer Freeman & Co., had gone about as far as it could without further investments in technology and expansion needed for reforms that are coming in both Medicare and Medicaid.