Scholars are generally agreed that the three main extant manuscripts of the Libro de buen amor represent two redactions of the poem: MSS.G and T are derived from the original version of 1330, and MS.S from the author's revised version of 1343. Evidently the revision effected in 1343 consisted of adding considerable material to the original version and eliminating a lesser amount. However, because the extant MSS are incomplete, it is impossible to ascertain precisely what material was eliminated and what was added in the revision. This revised text of 1343 can indeed be established with almost complete accuracy by taking the whole of S and supplying the contents of pages torn out of S or its ancestors, by means of G and T, which fortunately preserve substantially all the missing material. But unfortunately this procedure cannot be reversed to establish the contents of the 1330 version. To be sure, the lacunae in G-T can be identified, partly by the examination of the MSS and partly by comparison with S; and in certain cases, where material identifiable as a fraction of an episode is wanting, we can safely restore the gap with the text of S. But this procedure is not reliable where there is a major lacuna in G–T. One of these is the gap from st. 139b to st.329. In this paper I hope to shed possible light on the problem by putting forth the hypothesis that, where this gap occurs in MS.G, eleven stanzas from S (140–150) could not have figured in the 1330 version, but are rather an interpolation of the 1343 revision.