Buccal-cell-derived DNA collected by a 'swish and spit' technique and blood-derived DNA were compared for ease of collection, participant acceptance, utility and accuracy for HLA class II DR typing by the polymerase chain reaction with sequence-specific primers (PCR-SSP). The HLA class II DR typing results determined from DNA extracted by proteinase K digestion followed by phenol/chloroform extraction and ethanol precipitation from buccal cells and blood cells were identical for all subjects we studied (n = 10). Class II typing by PCR-SSP using DNA extracted from buccal cells stored at -20, 4, 25 or 37 degrees C for 1 week was successful. The samples can be collected without medical supervision and are not affected by exposure to a variety of temperature conditions for up to 1 week. The stability of the buccal cell specimens to these extreme conditions demonstrates the utility of this 'swish and spit' technique for collecting nucleated cells for geographically dispersed large-scale population studies. Buccal-cell-derived DNA collected by a simple 'swish and spit' mouthwash technique is an excellent and practical substitute for blood-derived DNA.