It has previously been reported that MHC class I-deficient RBL-5 and YAC-1 lymphoma sublines show an enhanced NK sensitivity in vitro. In the present study such lymphoma variants were found to have different defects in H-2 biosynthesis such as 1) reduced beta 2-microglobulin and H-2 H chain transcription, 2) block in beta 2-microglobulin translation, and 3) impaired association between beta 2-microglobulin and H-2 H chains. For lines with the latter two defects, the results suggested that a major part of the H chains were arrested before the middle Golgi complex as indicated by their failure to undergo carbohydrate side chain trimming. The data suggest that NK sensitivity can be directly influenced by the membrane expression of MHC class I gene products of tumor targets because three independent molecular defects, all interfering with the cell surface H-2 expression, gave rise to NK-sensitive phenotypes. These variant lines will also be useful tools for studies of H-2 glycoprotein assembly and transport and for Ag presentation to CTL.