We have examined the MLR reaction in two sets of recombinants that differ in the I-J subregion. In both cases, significant stimulation was mediated by antigens controlled by genes in the I-J subregion. This stimulation was inhibitable by the addition of the culture of antisera directed against the I-J gene products on the stimulator cell. The specificity of this inhibition was shown by specific blocking of the relevant gene product on F1 hybrid stimulator cells. MLR stimulation was also eliminated by pretreatment of the stimulator population with anti-I-J sera plus complement. Pretreatment of F1 hybrid stimulator T cells with anti-I-J sera directed against either parental I-J product in the presence of complement, completely eliminated stimulation, indicating that there is no allelic exclusion of the relevant I-J products. Pretreatment with an anti-I-E/I-C serum and complement also eliminated stimulation, suggesting that the stimulating T cells express both I-J and I-E/I-C subregion products. This assay offers a potentially more direct and practical method for serological detection of the I-J products.

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