C.B-17 scid mice were transplanted ip with human peripheral blood mononuclear cells. Some transplanted mice were immunized ip with birch pollen. Levels of total and birch pollen-specific human IgE were measured in serum. Low levels of total human IgE were detected after 9 days. 20 days after transplantation 12 out of 12 mice transplanted with cells from a birch pollen allergic donor showed higher but variable levels of total human IgE, and a low level of birch pollen specific IgE was detected in one mouse that had not been immunized. Immunization with birch pollen did not influence the level of total human IgE. In a different experiment, mice transplanted with cells from a donor with a high level of total IgE, but without birch pollen allergy, developed high levels of total IgE 49 days after transplantation, whereas no birch pollen specific IgE was found (two immunized, two non-immunized mice). In contrast, two of four mice given cells from two donors with birch pollen allergy, but with lower levels of total IgE, produced birch pollen-specific IgE after immunization. Thus, measurable levels of total human IgE were spontaneously produced in most scid mice transplanted with human peripheral blood mononuclear cells. Detectable levels of birch pollen specific human IgE appeared to be produced in occasional non-immunized scid mice transplanted with cells from allergic donors. In mice given cells from some allergic donors, half the mice produced human birch pollen specific IgE after immunization.