Twenty lymphoblastoid cell lines were typed with 38 antisera defining the 13 groups of the Merrit B-cell alloantigenic system, and results were compared with those of a 20-member CLL cell panel. While overall pattern of Merrit groups for the two types of cell population was similar, some sera in a number of groups showed unexpected negative cytotoxic reactions with lymphoid cells compared to other sera of the same group. However, in a number of instances, these negatively reacting cells could still specificity absorb out tb Merrit antibodies, suggesting that lymphoblastoid lines may show a higher incidence of the CYNAP (cytotoxicity-negative-absorption-positive) phenomenon than CLL cells. Twelve of the lymphoblastoid lines were HLA-D homozygous and in addition, each cell line displayed positive reactions for no more than 2 of the 13 Merrit groups, while 4 of the 8 heterozygous lines were positive for 3 of the 13 groups. These findings indicate that HLA-D-homozygous cell lines are antigenically simpler than the heterozygous lines for the Merrit system and suggest that HLA-D-homozygosity may correlate with homozygosity for at least one of the Merrit loci.