Yes — but the combined work must be released under the more restrictive license.
GNU Lesser GPL v2.1 is a file-level (or library-level) copyleft license. You can include GNU Lesser GPL v2.1 files in a ISC License project; the GNU Lesser GPL v2.1 files retain their copyleft obligation (changes must be released), while the project at large can stay ISC License. Keep file boundaries clear.
| License | Family | Patent grant |
|---|---|---|
| GNU Lesser GPL v2.1 (LGPL-2.1) | weak-copyleft | No (implicit at most) |
| ISC License (ISC) | permissive | No (implicit at most) |
GNU Lesser GPL v2.1: Library copyleft: dynamic linking permitted from non-LGPL code without triggering full copyleft.
ISC License: Functionally equivalent to MIT; Internet Systems Consortium's house license.
If you found this page because you're trying to figure out whether shipping a particular dependency is safe, the answer above is a starting point — not a substitute for reading the actual licenses or talking to a lawyer when stakes are high.
LicenseHound walks every transitive dependency in your repo, maps each to its SPDX license, and flags pairs like this one in PR comments. The CLI is free; the team dashboard is paid.