Further evidence for quantitative variation in H-2-antigen expression in F1 hybrids is described. Spleen cells from nine different F1 hybrids were compared with parental strain cells for differences in H-2K- and H-2D-antigen expression by quantitative absorption analysis and in an125I-radioimmunoassay using anti-H-2 sera. Some K and D antigens are expressed in two to three times smaller amounts in F1 hybrids, while others are approximately equally expressed. There was no evidence for eitherI-region control or parental influence in the genetic control of H-2-antigen expression. Expression of K and D antigens in an F1 hybrid (compared with parental cells) is dependent on theK andD allelic composition of the hybrid, in that some alleles tend to dominate over others. In four different F1 hybrids,K k was more strongly expressed thanK b , while, in other F1 hybrids,K d predominated over bothK k andK b . Similarly,D b dominated overD d in five different F1 hybrids, while no obvious dominance pattern was found in F1 hybrids carryingD k .