A simple method for obtaining DNA from serum and plasma is described. Using appropriate primer pairs the polymorphic segments of HLA class II genes were amplified by the polymerase chain reaction (PCR) from this DNA, and typed using allele-specific oligonucleotide hybridization. When compared with DNA obtained from peripheral blood lymphocytes, the efficiency of the PCR was only minimally compromised and could be augmented by increasing the number of amplification cycles and/or by the addition of glycerol to the reaction mixture. This method serves as a reasonable alternative when no other source of DNA is available.