\s=b\ Ninety adults with "mountain cedar" pollen allergy were started on coseasonal immunotherapy regimens in December 1979. At season's end, there were 20 whose starting antigen dose was based on serial dilution end point titration (SDET), 1:2,500 or 1:12,500 weight by volume (w/v); 28 in a "traditional" group started at 1:62,500 w/v; and 28 who had been double-blind placebo control subjects. Injections twice weekly were pushed aggressively toward maintenance. The SDET group received 11 times more antigen than the traditional group; both showed a trend for fewer symptoms than the placebo group. As compared with the other groups, SDET subjects had substantially blunted total and specific IgE responses in season, lower specific IgE level at season's end and less shift of skin end point toward higher dilutions. The findings demonstrate that substantial immunologic effects are possible in the first months of aggressive coseasonal immunotherapy.