The New York-based company previously raised $97.9 million in venture funding. The prior round, a Series C worth $75 million led by Goldman Sachs and DST Global, valued the company at $700 million. DST Global, the fund run by Russian entrepreneur Yuri Milner, appears to be participating the latest round as well.

Founded in 2007, ZocDoc has grown to 550 employees, offering its services in 2000 cities with plans to go nationwide by the end of the year. The company attracted the attention of traditional healthcare companies, including Aetna, which reportedly offered to acquire the company for $300 million in 2013.

The company declined to comment.

Earlier this year, CEO Cyrus Massoumi tamped down IPO rumors by noting that ZocDoc was in no hurry to go public. ZocDoc has five million patients using its system to book doctors’ appointments. It has never revealed how many doctors pay the company a monthly fee for its listing services, because it does not want to reveal how much revenue it earns. In February, ZocDoc moved its CFO of almost six years and fifth employee, Netta Samroengraja, to the role of Chief People Officer, bringing in Joseph Holland, previously CFO WeddingWire, for the role. Massoumi told Fortune at the time that the move was Samroengraja’s decision, and came two years in the making.

This month ZocDoc unveiled a new subscription service for employers called ZocDoc for Business, which offers more personalized search results and services.