Tests show the monoclonal anti CD4 antibody (mAb) MT310 recognizes the gp120 binding site on CD4 as part of its mechanism for strongly inhibiting human immunodeficiency virus type 1 (HIV 1) infection of CD4 + T cells. In competition tests, mAb MT310 and mAb Leu3a (an anti CD4 mAb recognizing the gp120 binding site) all inhibited gp120 binding to CD4 + T lymphocytes, while mAb MT405 did not. This result suggests that MT310, like Leu3a, recognizes the gp120 binding site on CD4. To further confirm whether MT310 recognizes the gp120 binding site on CD4, we prepared rabbit anti idiotypic antisera (Ab2) against MT310 (Ab1). The anti idiotypic antisera against MT310 inhibited binding of MT310 and Leu3a to human CD4 + T lymphocytes, but did not block binding of MT151 with the second domain of CD4, while rabbit anti idiotypic antisera to MT151 could block binding of itself to these cells, but could not inhibit the binding of MT310 and Leu3a, further indicating that MT310 recognized the gp120 binding site on CD4.  Tests show the monoclonal anti CD4 antibody (mAb) MT310 recognizes the gp120 binding site on CD4 as part of its mechanism for strongly resistant human immunodeficiency virus type 1 (HIV 1) infection of CD4 + T cells. In competition tests, mAb MT310 and mAb Leu3a (an anti CD4 mAb recognizing the gp120 binding site) all inhibited gp120 binding to CD4 + T lymphocytes, while mAb MT405 did not. This result suggests that MT310, like Leu3a, recognizes the gp120 binding site on CD4. recognizes the gp120 binding site on CD4, we prepared rabbit anti idiotypic antisera (Ab2) against MT310 (Ab1). The anti idiotypic antisera against MT310 inhibited binding of MT310 and Leu3a to human CD4 + T lymphocytes, but did not block binding of MT151 with the second domain of CD4, while rabbit anti idiotypic antisera to MT151 could block binding of itself to these cells, but could not inhibit the binding of MT310 and Leu3a, further indicating that MT310 recogni zed the gp120 binding site on CD4. 