The case highlights the increasingly important role of online materials in higher education. M.I.T. and Harvard have extensive materials available free online, on platforms like YouTube, iTunesU, Harvard@Home and MIT OpenCourseWare. In addition, the two universities are the founding partners of edX, a nonprofit that offers dozens of MOOCs, or massive open online courses, free to students around the world.

The complaints say Harvard and M.I.T. violated both the Americans With Disabilities Act and the Rehabilitation Act of 1973, and seek a permanent injunction requiring them to include closed captioning, which provides a text version of the words being spoken, in their online materials. Despite repeated requests by the association, the complaints say, the two universities provide captioning in only a fraction of the materials, “and even then, inadequately.”

The lawsuits, filed by the National Association of the Deaf, which is seeking class-action status, say the universities have “largely denied access to this content to the approximately 48 million — nearly one out of five — Americans who are deaf or hard of hearing.”

Bill Lann Lee, the Oakland, Calif., lawyer who represented the association along with lawyers from several disability rights groups, said the association thought that because Harvard and M.I.T. had been leaders in putting university content online, a change in their practices would have an impact on other universities’ policies.

The federal government has already moved to ensure that blind students will not be left out by the adoption of electronic readers; it is now taking action to ensure that deaf students have access to captioned materials.