Platelet GPIbα and GPIX were precipitated by antibody JH in the presence (lane 4), but not in the absence (lane 3), of ranitidine. Monoclonal AP1, specific for GPIbα precipitated GPIbα and GPIX both in the presence (lane 6) and absence (lane 5) of drug. No bands were precipitated by normal plasma (lanes 1 and 2) or by normal mouse IgG (lane 7). GPIbα and GPIX were demonstrated by Western blotting with monoclonals MBC142.11 and GR-P, under reducing and nonreducing conditions, respectively.