Lymphocyte stimulation, as determined by incorporation of thymidine, to rye grass extract in twenty-three children with bronchial reactivity to rye grass and to house dust mite, did not differ significantly from four children with reactivity to house dust mite alone, or from nine children with asthma but without a bronchial response to these allergens. Sixteen children underwent hyposensitization with rye grass extract or treatment with placebo. There was no consistent effect of hyposensitization on the lymphocyte stimulation indices to rye grass. A decrease in lymphocyte responsiveness occurred to rye grass and to house dust mite after the grass pollen season but was not statistically significant. Analysis of changes in lymphocyte responsiveness to both house dust mite and rye grass of the children most highly sensitized to rye allergen, showed that the lymphocyte responsiveness to rye grass fell during the pollen season (P less than 0.05) but this effect was not seen with house dust mite. The study suggests that a decrease in lymphocyte responsiveness to rye grass allergen in children with large amounts of anti-rye IgE antibodies is antigen specific and may be seen following seasonal exposure.