New Mountain Capital LLC has purchased Stoughton, Mass.-based Medical Specialties Distributors (MSD) for an undisclosed price.

The deal marks an exit for Water Street Healthcare Partners, which acquired MSD in 2010. The Chicago private equity firm expanded the company into the growing home-infusion therapy market, which includes services that provide patients with intravenous medication from their own homes.

Water Street also grew MSD through dealmaking. In June, MSD acquired Medical Technology Resources LLC , a Columbus, Ohio-based  provider of infusion pumps and biomedical and billing services.

Water Street’s sale of MSD is the PE firm’s fourth divestiture in recent months. In October, Water Street sold pharmaceutical services company AAIPharma Services Corp. to Cambridge Major Laboratories. Later that month, the firm divested two laboratory services companies — ConVerge Diagnostic Services PLUS Diagnostics — to Quest Diagnostics and Miraca Life Sciences, respectively.

New York-based New Mountain Capital manages roughly $9 billion in capital commitments.