Since you state your contribution is a few percent, I'd say it's fair. Probably not the nicest situation, though.
Here's what you can do. If you're still in one of these labs, look around online, find a local conference or maybe an undergrad research day at your school, and ask your PI if you can present your results there. At least you'll get a poster out of the deal.
If you're not still working, then be more up-front about your desire to be in a pub when you apply for a job. Then back up your statement with a contribution. That means doing more than running assays. It means coming up with a question, running it by your PI. Maybe designing the experiments to test your question. Or maybe volunteer to write up one of your PI's papers. Offering to finish up a series of half-done experiments is an easy way to get authorships and do your PI a big favor.