The following is from the Commonwealth of Massachusetts Archives' web site:
Massachusetts officials started recording the names of immigrants who arrived by ship in January of 1848, a procedure which continued until July of 1891, when federal records-keeping programs superseded those of the state. Although immigrants arrived at numerous Massachusetts ports, the Archives holds manifests for BOSTON ONLY. These are arranged chronologically according to the date when the ship arrived in port. The Archives holds the original manifests as well as the only microfilm copy available of these Passenger manifests. This collection has been cataloged as HS3.02/1990X (Registers of passengers arriving in Massachusetts ports) in the finding aids of the Massachusetts Archives.
The Archives has an alphabetical card index covering the same years as the registers and providing the same information. This includes the name, age, sex and occupation of the immigrant; the country of birth, last residence and the passenger list number. Also included is the name of the ship, and the date of its arrival in Boston.
Over one million immigrants came through the Port of Boston during this time period. Through the Archives Volunteer program, volunteers are entering this information into a computerized database. Once a surname is entered into the database it goes through an editing process and when completed it will appear on this website. This database will be updated periodically.
If the Surname you are interested in does not appear in this database you are welcome to come into the Archives and request a search through the card index. Some names may not be available because they are being processed.
Keep in mind that the project is not yet completed. You can search the names that are already available in the database at http://www.sec.state.ma.us/arc/arcsrch/PassengerManifestSearchContents.html