Seven murine anti-H-2 and three nonimmune mouse sera were tested for cytotoxicity toward B and T lymphocytes from a panel of human donors. One group of sera, including two anti-H-2.33 sera, exhibited cytotoxicity directed exclusively toward human B but not T cells from all donors. Absorption studies on human lymphoblastoid cell lines (LCLs) of B or T cell origin corroborated these findings. Some nonimmune sera showed similar characteristics, indicating that the observed reactions were not attributable to cross-reactivities between mouse H-2K or D specificities and human antigens coded by theHLA-A, B,orClocus. Another set of mouse sera (anti-H-2.28b and anti-H-2.31) was highly cytotoxic to both B and T cells of some donors but not of others, suggesting that activity in these sera may arise from cross-reactions between mouse and human specificities. A third set of anti-H-2 as well as normal mouse sera showed only background cytotoxicity when tested on human cells.The ability of the B cell cytotoxic mouse sera to block the human mixed lymphocyte culture reaction (MLR) was compared to that of appropriate human alloantisera with exclusive B cell activity or a rabbit serum raised against human B cells. None of the mouse sera resulted in a significant reduction in the human MLR, whereas the human alloantisera as well as the rabbit antiserum caused a significant amount of blocking at several dilutions beyond their highest cytolytic titer.In an extensive search for cross-reactivity between specificities of the mouse and human major histocompatibility complex (MHC), Ivaskova et al. (1) found that many congenic H-2 sera react with almost all human cells if the serum dilutions are sufficiently low. Associations between someHLAlocus antigens (mainly A2, B7, and B27) and certain of the H-2 sera can reportedly be detected only when end point titrations are carried out. The authors point out that there is considerable variability in the strength of reactions of human cells with the mouse alloantisera and that reproducibility of typings is not as good as with HLA alloantisera. This may indicate that some of the antibodies in the H-2 antisera may actually be reacting with subpopulations of human lymphocytes. The proportions of B and T cells from different donors seem to vary from bleed to bleed. This variability could result in different end titration points in sera containing mostly B cell antibodies. The reactivity of the HLA specificities with well defined typing sera usually varies little. This is to be expected, since HLA antigens are present on all lymphocytes. Therefore, some of the differences in reaction strengths observed when donors carrying similar HLA antigens are tested with heterologous antisera may be attributable to reactions involving only certain lymphocyte subpopulations.A preliminary investigation in this laboratory indicated that human LCLs, with B cell characteristics are much more sensitive to H-2 alloantisera than those classified as T cells. The present study demonstrates that some mouse sera exhibit cytotoxic patterns toward separated human B and T cells that closely resemble those of xenoantibodies produced against human B cell lines. The effect of the mouse sera on the human MLR is also investigated. The blocking activity of the mouse sera is, however, much weaker than that of xenoantibodies or human alloantibodies recognizing DRw specificities.