Rat basophil leukemia (RBL) cells were sensitised with varying proportions of monoclonal IgE anti-ovalbumin (OVA) and anti-DNP antibodies, and serotonin release was measured after challenge with aggregated OVA or dinitrophenylated human serum albumin (DNP-HSA). Highly aggregated OVA was shown to provoke the degranulation of RBL cells that had been sensitised with an IgE preparation containing 2% IgE anti-OVA antibodies. Highly substituted DNP32-HSA induced degranulation of RBL cells sensitised with just 0.5% antigen-specific IgE. When cells were sensitised with high percentages of specific IgE, maximum degranulation was seen at concentrations of 2 micrograms/ml (aggregated OVA) and 50 ng/ml (DNP-HSA), while moderate degranulation was still seen at antigen concentrations as low as 50 and 2 ng/ml, respectively. Low-molecular weight aggregates of OVA and low-valency DNP4-HSA only stimulated degranulation when high percentages of RBL Fc epsilon receptor were occupied by antigen-specific IgE. The sensitising abilities of two anti-DNP monoclonal antibodies of differing affinities were compared. When challenged with low-valency antigen, only cells sensitised with the higher-affinity monoclonal antibody exhibited moderate levels of degranulation. Degranulation required exposure to high antigen challenge doses (5 micrograms/ml). Cells sensitised with either monoclonal antibody responded strongly when challenged with a wide range of concentrations (1-250 ng/ml) of high-valency DNP32-HSA, although greater sensitivity was always seen with the higher-affinity antibody. These results suggest that antigen valency is a critical parameter for mast cell function, and that low-affinity antibody may be capable of sensitising mast cells to high-valency antigen.