Monday's games between the Philadelphia Phillies and New York Yankees and the Miami Marlins and Baltimore Orioles have been postponed, MLB has confirmed.
The Marlins played a three-game series in Philadelphia last week but delayed their return home to Miami amid fears of a coronavirus outbreak in the team.
There have been reports of as many as 14 individuals in the Marlins camp contracting COVID-19.
MLB did not release details of the number of confirmed cases but said it was waiting on the results of "additional testing".
"Tonight's scheduled games between the Miami Marlins and the Baltimore Orioles at Marlins Park and the Philadelphia Phillies and the New York Yankees at Citizens Bank Park have been postponed while Major League Baseball conducts additional COVID-19 testing," the league said.
"The members of the Marlins' travelling party are self-quarantining in place while awaiting the outcome of those results.
"Major League Baseball has been coordinating with the Major League Baseball Players Association, the Marlins, the Orioles, the Marlins' weekend opponents, the Phillies, and club medical staffs, and will continue to provide updates as appropriate."
News of an outbreak in the Marlins team came after starter Jose Urena was scratched just hours before Sunday's game without reason.
The 2020 season is less than a week old after Opening Day was postponed from March due to the coronavirus pandemic, with the schedule subsequently reduced to 60 games.
While the NBA, NHL and MLS have adopted 'bubble' systems to keep the teams contained between games, MLB players have continued to travel the country.
The number of confirmed COVID-19 cases in the United States continues to grow, with more than four million positive tests across the nation.