By lab professor you mean the principal investigator of a research lab you were involved with? or some one who taught your lab? people who usually teach labs are Masters or PhD students.
But you did not have this professor for the lecture portion of the course, so with that said his letter may not fulfill the science professor requirement for most schools. But get his letter anyways, you can have up to five letters. Do you have a review committee at your undergrad who can write a composite letter on your behalf?