When elicited in the skin of mice, either IgE-dependent immediate hypersensitivity reactions or T cell-dependent contact sensitivity (CS) reactions result in local extravasation of [125I]fibrinogen and deposition of [125I]fibrin. However, these two types of reaction differ in kinetics and in requirement for IgE, mast cells, or T cells. In the present study, we investigated the kinetics and magnitude of [125I]fibrin deposition in combined IgE-dependent and CS reactions elicited simultaneously at the same site and compared the results with those obtained when the two reactions were elicited at separate sites. We found that [125I]fibrin deposition in pure IgE-dependent reactions was greater at 2 or 6 h after challenge than at 24 h, but that significant fibrin deposition persisted at those sites 24 h after challenge. In CS reactions, [125I]fibrin deposition was detected as early as 2 h after challenge, indicating that fibrin deposition accompanies the "early component" of CS detected by Van Loveren et al. with the use of measurements of tissue swelling. But much more [125I]fibrin deposition was present in CS reactions at 24 h than at 2 or 6 h after Ag challenge. When IgE-dependent and CS reactions were elicited at the same site, [125I]fibrin deposition at early intervals (2 to 6 h) after challenge was increased three- to 25-fold compared with that seen in isolated CS reactions, but at 24 h the results in the combined reactions were virtually identical to those in CS responses. Studies in genetically mast cell-deficient and congenic normal mice indicated that mast cells were required for expression of the IgE-dependent augmentation of [125I]fibrin deposition observed at early intervals in combined IgE-dependent and CS reactions, but not for the [125I]fibrin deposition associated with "pure" CS reactions. These findings indicate that the net effect of IgE-dependent mast cell activation on CS responses is to increase the fibrin deposition associated with these responses, but this effect is appreciated only at early intervals after elicitation of the reaction.