The optimal conditions for the extraction and isolation of immunoglobulins from the cell membrane (M-Ig) of T and B lymphocytes radioiodinated with lactoperoxidase were studied. A combination of 10M-urea, 1.5 M acetic acid and the non-ionic detergents Triton X-100 or Nonidet P-40 (NP-40) was found to overcome the tendency for membrane proteins to precipitate after extraction in urea-acetic acid alone. However, detection of T cell Ig was more dependent on detergent concentration than B cell M-Ig. Less T cell Ig was detected when the detergent concentration was increased from 0.1% to 1.0%, whereas the amount of B cell M-Ig was not affected by higher detergent concentrations.