Data from two field trials of influenza vaccine were examined for an association between vaccination time and subject response. Both were conducted on adults and involved the same antigens and dose via intramuscular injection. In one study (Princeton) a diurnal pattern in antibody response to the antigen A/Philippines, but not to A/Chile or B/USSR, was detected after the first vaccination administered during summer 1984, but not after the second revaccination given during summer 1985. In a second study (Houston) conducted during autumn 1985, no diurnal pattern in antibody response was detected for any of the antigens assessed. No diurnal pattern in systemic reactions was observed. Previously vaccinated subjects of both studies more commonly experienced local reactions of arm redness, hardness, and soreness after afternoon versus morning (p > 0.05) injection upon revaccination, both before and after adjustment for possible confounders of age and gender.