Over the past decade, hypotheses regarding the site of Ir gene action have changed considerably (1, 2). The original, widely held concept (3) that/r genes were expressed in T cells gave way to the belief that they were expressed in antigen-presenting cells (APC) 1 of the macrophage-monocyte-dendritic cell lineage, or at the very least, in T cell-APC interactions. The B cell, which actually produces the antibodies, was given a relatively minor role. Some authors (4) presented evidence that the B cell played no role at all, that everything could be explained solely on the basis of T cell interaction with the APC. Others (5, 6) presented evidence that the B cell played a role parallel to that of the APC in that T cells were genetically restricted to interact with high but not low responder B cells as well as APC. This controversy was at least partially resolved by the discovery (7, 8) that T cell help for Lyb5-B cells was genetically restricted, whereas that for Lyb5 + B cells was not. These and related studies led to the hypothesis that, what the T cell saw first on the APC (a certain combination of antigen and Ia), it must see reproduced on the B cell to mediate help (9, 10). Most of these studies involved synthetic repeating polypeptide antigens, such as (T,G)-A--L.We have been studying the genetic control of the immune response to a natural globular protein antigen, sperm whale myoglobin (Mb), and have found that the in viva antibody response (11), the in vitro T cell response (12), and the in vitro antibody response (13) are all controlled by at least two/r genes mapping in distinct subregions, I-A and I-C, of the H-2 complex. These genetically separable genes control responses to chemically distinct determinants on the same antigen molecule (12, 13). In the case of T cell proliferation, we found that the selection of which antigenic determinant was stimulatory for (high responder × low responder)F1 T cells depended on the /r genes of the strain from which the APC were obtained (14). Thus, the Ir gene restriction of T ceI1-APC interaction led to what had been described by Rosenthal et al. (3) as "determinant selection." However, we wished to explore the cellular interactions involved in regulating antibody production rather than T cell proliferation. To do so, we recently developed (13) an in vitro culture system in which we can measure soluble Mb-specific antibody * Present address: