A Seyfert-like nucleus concealed in BL lacertae?

We present a series of new spectroscopic observations of BL Lacertae. The o bservations were obtained over a period of 30 months and sample the violent continuum flare of 1997 June-August. The equivalent width of the recently discovered broad H alpha line varies approximately inversely with the optic al continuum flux, implying that the broad-line-emitting gas does not respo nd significantly to large variations in the beamed synchrotron continuum wh ich dominates the spectrum at optical wavelengths. The simplest and most pl ausible interpretation of this result is that the broad-line region in BL L acertae is 'unaware' of the synchrotron beam and is photoionized by another source, a likely candidate being a hot accretion disc. Alternatively, simi lar behaviour would be observed if the broad-line region is indeed photoion ized by the synchrotron beam but is predominantly matter-bounded so that it absorbs only a small fraction of the incident ionizing photon flux.