Quantitation of surface and total cell Ig obtained after lysis by detergent, urea-acid treatment, and freeze-thawing were determined on spleen cells, thymus cells, and spleen cells specifically depleted of B cells. A two- to four-fold increase in measurable Ig was found after cell lysis. All cell populations showed a similar increase in measurable Ig indicating that no discordantly large amounts of buried Ig determinants were associated with the surface of T cells. The lack of appreciable amounts of T cell Ig was confirmed by immunoprecipitation of radioiodinated cells. A theta-positive lymphoma was described which, when grown in culture, lacked detectable surface Ig but contained a receptor site for IgG. This resulted in appreciable amounts of surface IgG being associated with the tumor line when isolated from ascitic fluid of tumor-bearing mice or after preincubation of cultured cells with either heat-aggregated IgG or normal mouse serum.