Albumin was shown to be hidden component of mouse B and T lymphocyte plasma membranes. It was readily radiolabeled from within the plasma membrane by a lipophilic, photoactivated reagent (125I-iodonaphthylazide) but not from the cell exterior by lactoperoxidase-catalyzed iodination; it was not detected by immunofluorescence on intact cells. The function of this cryptic lymphocyte membrane albumin is unknown at present. This cryptic albumin component was discovered during immunoprecipitation experiments using anti-Ig reagents. It is considered of general methodologic significance that many antisera, both native and rigorously Ig-absorbed (both positively and negatively), contained such contaminating activity. The possibility is raised that such contaminating activity may be involved in some reported findings of albumin-size "Ig-like heavy chains" in both the B and T lymphocyte lineages.