A patient carrying hepatitis B antigen with both d and y specificities on the same particles (diplotypic HB Ag) has been followed over a period of two years, with examination of repeated blood samples.Titration studies on these sera consistently showed a stronger reaction and higher titer of specificities a and w, as compared with specificities d and y. These relationships differed from those seen in sera containing HB Ag of the usual ‘simple’ kind, whether of subtype D or subtype Y, in which the reactions of a and d, or a and y, paralleled each other closely. It is hypothesized that the most likely explanation for the appearance of this diplotypic HB Ag is a double infection with hepatitis B viruses of subtypes D and Y, resulting in a combination of the adw and ayw genomes, and the formation of a new viral genome possessing genes for both the d and the y specificities.