Growth of human MM cells within SCID-hu mice triggers human monoclonal Ig secretion. ARH-77 and OCI-My5 MM cells were injected directly into the left human fetal bone implant in SCID-hu mice, and sera samples were taken from these mice 4, 8, and 12 weeks later. Human IgG (▪) in sera from mice injected with ARH-77 MM cells and human IgA (▧) in sera from mice injected with OCI-My5 MM cells were quantitated by ELISA.