A quantitative ELISA was developed for measurement of anti-yeast antibodies. The finding of raised levels of yeast antibodies in Crohn's disease was confirmed and the data suggest that this may be related to the presence of disease in the small bowel, although this latter observation did not reach statistical significance. Patients with chronic liver disease also had higher antibody levels than controls but less markedly so than in Crohn's disease. When sera were tested, using a similar assay, for antibodies to bovine casein, no difference was found between controls and the Crohn's or liver disease group. The response of peripheral blood mononuclear cells (PBMC) to S. cerevisiae extract was examined using a proliferation assay measuring uptake of tritiated thymidine. Cells from normal controls demonstrated dose-dependent proliferation, the time-course of which resembled that obtained with known recall antigens. Following separation of cell populations by rosetting with sheep erythrocytes, the responding cells were shown to be T-lymphocytes and the magnitude of the response was sensitive to the number of antigen-presenting cells present in the culture. When immunomagnetic beads were used to further separate T-cells, by positive selection, into highly purified CD4+ and CD8+ populations, responsiveness to yeast co-separated with the CD4+ subset. Negative selection of CD4+ cells according to phenotypic expression of the RO and RA isotypes of the CD45 molecule was used to demonstrate that responsiveness was greater in, but not exclusively confined to, CD450RO cells. Classical limiting dilution analysis of peripheral blood T-cells responsive to yeast resulted in high values for the precursor frequency, although the experimental data could not be shown to conform to single-hit kinetics. It was possible, by sequential stimulation in long term culture, to obtain populations of cells which were uniquely responsive to yeast but unresponsive to other recall antigens. Proliferation responses, to yeast, of PBMC from Crohn's patients were no different from those in normal subjects.