Cross-reactive influenza virus–specific antibody-dependent cellular cytotoxicity (ADCC)–activating antibodies are readily detected in healthy adults. However, little is known about the kinetics of these ADCC responses. We used retrospective serial blood samples from healthy donors to investigate this topic. All donors had ADCC responses against 2009 pandemic influenza A(H1N1) virus (A[H1N1]pdm09) and avian influenza A(H7N9) virus hemagglutinins (HAs) despite being seronegative for these viruses in standard hemagglutination inhibition and microneutralization serological assays. A(H1N1)pdm09 exposure did not boost ADCC responses specific for H7 HA antigens. H7 HA ADCC responses were variable longitudinally within donors, suggesting that these cross-reactive antibodies are unstable. We found no correlation between ADCC responses to the H7 HA and either influenza virus–specific immunoglobulin G1 concentration or age.