The law in California does not always allow discovery responses in a completed case to be used in a newly filed case.

An adverse party may "use for any purpose, a deposition of a party to the action, or of anyone who at the time of taking the deposition was an officer, director, managing agent, employee, agent, or designee under Section 2025.230 of a party."  (Code of Civil Procedure Sec. 2025.620.)  Read literally, one can conclude that deposition testimony from a previous case can be used against a party in a new case.  Certainly, exceptions to the California hearsay rule would allow it: Admission of Party (Evidence Code Sec. 1220), Declaration Against Interest (Evidence Code Sec. 1230), and Former Testimony (Evidence Code Sec. 1290).

Responses to written discovery in a previous case are another matter.  Clearly, responses to Requests for Admissions may only be used in the case in which the responses were given.  (Code of Civil Procedure Sec. 2033.410(b).)  However, at least arguably, responses to Interrogatories drafted in a closed case may be used in a newly filed case.

In regard to the use of responses to Interrogatories, Code of Civil Procedure Sec. 2030.410 states: "At the trial or any other hearing in the action, so far as admissible under the rules of evidence, the propounding party or any party other than the responding party may use any answer or part of an answer to an interrogatory only against the responding party."  Do the words "the trial or any other hearing in the action" mean that the responses to interrogatories can only be used in that action?  If the legislature did not intend for those responses to be used in a new action, why did it not have a complete prohibition similar to the one for Requests for Admissions?  If the responses to interrogatories from a closed case may be used in cases filed after that case then a "party" may use the responses against the responding party only.

The best practice is to not rely on discovery gathered in an old case as evidence in a new case.  Arguably, even deposition testimony from an old case cannot be as freely used in a new case as it could have been used in the case in which the deposition was taken.