Cochlear gain reduction via efferent feedback from the medial olivocochlear bundle (MOCB) is frequency specific (Guinan, 2010). The present study with humans used the Fixed Duration Masking Curve (FDMC) psychoacoustical method (Yasin et al., 2013a,b, 2014) to estimate the frequency specificity of the efferent effect at the cochlear level. The combined duration of the masker-plus-signal stimulus was 25 ms, within the efferent onset delay of about 31-43 ms (James et al., 2002). Masker level (4.0 or 1.8 kHz) at threshold was obtained for a 4-kHz signal in the absence or presence of an ipsilateral 60-dB SPL, 160-ms precursor (200-Hz bandwidth) centred at frequencies between 2.5 and 5.5 kHz. Efferent-mediated cochlear gain reduction was greatest for precursors with frequencies the same as, or close to that of, the signal (gain was reduced by about 20 dB), and least for precursors with frequencies well removed from that of the signal (gain remained at around 40 dB). The tuning of the efferent effect filter (tuning extending 0.5 to 0.7 octaves above and below the signal frequency) is within the range obtained in humans using otoacoustic emissions (OAEs) (Lilaonitkul and Guinan, 2009; Zhao and Dhar, 2012). The 10-dB bandwidth of the efferent-effect filter at 4000 Hz was about 1100 Hz (Q10 of 3.5). The FDMC method can be used to provide an unbiased measure of the bandwidth of the efferent effect filter using ipsilateral efferent stimulation.