The SCID (Severe Combined Immune Deficiency) mutation causes two DNA recombination deficiencies: an aberrant joining of V(D)J immunoglobulin gene elements and a failure to perform efficient repair of DNA double-strand breaks. A recently established cell-free assay for DNA transfer (DTA) was applied to study nuclear extracts from normal and SCID-derived cells. The recombination deficiency was reflected in the cell-free system: SCID lymphocyte and fibroblast extracts showed reduced levels of DTA activity on a variety of DNA substrates. Analysis of nuclear extracts prepared from wild- type thymocytes and B cells representing different stages in lymphocyte ontogeny revealed the highest activities at the most immature stages. With progression of development, DTA activity decreased. Corresponding to their early developmental arrest, V(D)J rearrangement-incompetent RAG-2(-/-) lymphocyte extracts show high DTA activity. In contrast, extracts from SCID early lymphocytes express very low DNA transfer activity. Induction of V(D)J rearrangement in vivo in a normal preB cell line lead to a co-induction of the cell-free recombination activity. This indicates a development stage specificity of cell-free DNA recombination, which temporally parallels V(D)J recombination. A protein could be purified to near-homogeneity from wild- type thymocytes which stimulates the recombination activity specifically in SCID thymocyte and proB cell extracts. This protein, SRSP (SCID Recombination Stimulatory Protein), migrates as a single band of approximately 72 kDa in SDS-polyacrylamide gel electrophoresis.