In a double blind trial of nasal spray of sodium cromoglycate (DSCG) for hay fever, fifteen of twenty-three patients taking the drug were helped, compared with thirteen of twenty-three receiving placebo—a statistically insignificant difference. Serum levels of IgE were raised in thirty-eight of forty-four patients examined. There was no significant difference between preseasonal and seasonal serum IgE levels in either of the DSCG or the placebo group. Serum IgE levels were significantly higher in the patients showing peripheral blood or nasal secretion eosinophilia during the hay fever season. There was no significant difference in the frequency of nasal secretion eosinophilia between those taking DSCG and those taking placebo. It is concluded that DSCG spray does not affect serum IgE levels or eosinophilia of either peripheral blood or nasal secretions under the conditions applying in this trial. Neither was DSCG significantly better than placebo in controlling symptoms.