The entire Lta gene and the majority of the Tnf gene (to codon 113 in exon 4) were replaced by a neomycin selection cassette inserted by homologous recombination. No transcript or truncated protein was detected by a variety of assays including Northern blot analysis of concanavalin A stimulated splenocytes and ELISA of macrophage colony supernatants. Tnf activity was undetected in LPS stimulated homozygous mutant mice.