The present invention concerns therapeutic agents that modulate the activity of TALL-1. In accordance with the present invention, modulators of TALL-1 may comprise an amino acid sequence Dz.sup.2Lz.sup.4 wherein z.sup.2 is an amino acid residue and z.sup.4 is threonyl or isoleucyl. Exemplary molecules comprise a sequence of the formulae TABLE-US-00001 (SEQ. ID. NO: 100) a.sup.1a.sup.2a.sup.3CDa.sup.6La.sup.8a.sup.9a.sup.10Ca.sup.12a.sup.13a.s- up.14, (SEQ. ID. NO: 104) b.sup.1b.sup.2b.sup.3Cb.sup.5b.sup.6Db.sup.8Lb.sup.10b.sup.11b.sup.12b.su- p.13b.sup.14Cb.sup.16b.sup.17b.sup.18 (SEQ. ID. NO: 105) c.sup.1c.sup.2c.sup.3Cc.sup.5Dc.sup.7Lc.sup.9c.sup.10c.sup.11c.sup.12c.su- p.13c.sup.14Cc.sup.16c.sup.17c.sup.18 (SEQ. ID. NO: 106) d.sup.1d.sup.2d.sup.3Cd.sup.5d.sup.6d.sup.7WDd.sup.10Ld.sup.13d.sup.14d.s- up.15Cd.sup.16d.sup.17d.sup.18 (SEQ. ID. NO: 107) e.sup.1e.sup.2e.sup.3Ce.sup.5e.sup.6e.sup.7De.sup.9Le.sup.11Ke.sup.13Ce.s- up.15e.sup.16e.sup.17e.sup.18 (SEQ. ID NO: 109) f.sup.1f.sup.2f.sup.3Kf.sup.5Df.sup.7Lf.sup.9f.sup.10Qf.sup.12f.sup.13f.s- up.14 wherein the substituents are as defined in the specification. The invention further comprises compositions of matter of the formula (X.sup.1).sub.a-V.sup.1-(X.sup.2).sub.b wherein V.sup.1 is a vehicle that is covalently attached to one or more of the above TALL-1 modulating compositions of matter. The vehicle and the TALL-1 modulating composition of matter may be linked through the N- or C-terminus of the TALL-1 modulating portion. The preferred vehicle is an Fc domain, and the preferred Fc domain is an IgG Fc domain.