A simple, rapid micromethod for determining Factor VIII-related antigen is outlined. The simplicity of the method, using commercial antisera, prepackaged buffer, plastic plates that readily accept the gel without precoating with agarose, and standard equipment makes the procedure adaptable to the routine laboratory. The results give a straight-line graph to the 0.50 units/ml range, and values as low as 0.03125 U/ml can be determined using log-log graph paper. A second range of standards gives values from 0.10 to 0.750 U/ml when plotted on semilog graph paper. A permanent record can be made after a first rapid visualization process, which permits reading the plates within 15 min of the two-and-a-half-hour electrophoresis time.