Abstract: There are semilogarithmic (lg-) concentration-response sigmoids with slopes exactly representing the opposite asymmetry of that encountered in the case of a receptor reserve (reserve: large curvature in the left part of the curve and narrow curvature in the right part; opposite: narrow curvature in the left part of the curve and large curvature in the right part). In addition, the binding Kds of agonists are smaller than their EC50 in such curves. Using experimental concentration-response data a non-linear fitting model of a receptor "antireserve" was developed which is the topic of the present paper. This model reflects the possibility of a subthreshold preactivation of a signaling system. Hypothetically, only after exceeding this threshold a receptor-mediated response takes place.