Lymph node cells (LNC) from Lewis, BN, (Lewis × BN) F1, and (BN × F1) backcross rats were evaluated in culture for their ability to incorporate 3HTdR when stimulated by concanavalin A (Con A) or phytohemagglutinin (PHA) at concentrations ranging from 0.06 to 64 µg/ml. Lewis LNC responses were significantly higher than those of BN cells at optimal or lower concentrations of either mitogen, and responses of F1 LNC tended to be intermediate. There were no statistically significant differences between the responses of LNC from males and females. Each backcross LNC population was tested with BN anti-Lewis serum and by mixed lymphocyte reactivity with F1 cells to determine its genotype at the major histocompatibility locus of the rat (AgB or H-1). The mean Con A- or PHA-stimulated responses of LNC from the 7 of the 14 backcross animals which inherited the AgB1 or H-1l-haplotype characteristic of the Lewis strain were not higher than the mean response of LNC from their siblings who were homozygous for the major histocompatibility locus characteristic of the low responding BN strain. These results are compatible with multigenic control of the potential for thymus-cell DNA synthesis by lymph node cells from normal rats. No evidence was obtained for linkage of any of these genes to the major his tocompatibility locus of the rat.

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