Laminin is a heterotrimeric glycoprotein found in the perisinusoidal space of adult rat liver. The principal cellular source of laminin in liver is the lipocyte, with its three subunits measuring 324, 200 and 200 kD. The large subunit of lipocyte-derived laminin is distinct from the A subunit of murine laminin (440 kD); its size suggests that it represents a peptide, called M, recently cloned from human placenta. Using oligonucleotide primers derived from the human M-subunit cDNA, we amplified a 445-bp sequence encoding a fragment of M-laminin from adult rat lipocytes. The rat cDNA is 90% homologous to the human M-subunit cDNA and recognizes an mRNA in lipocytes measuring about 10 kb. M-subunit transcripts were identified only in lipocytes from normal adult liver; they could not be identified in hepatocytes, endothelial cells or Kupffer cells. Lipocytes were screened for M-subunit protein with a polyclonal M antiserum. Cells stained specifically for the M-subunit after 36 hr in primary culture; the protein was also identified in freshly isolated cells by means of immunoblotting. To determine whether lipocytes alter their expression of the laminin M subunit during liver injury, we monitored M-subunit mRNA in these cells at various intervals after carbon tetrachloride administration. M-subunit transcripts increased twofold within 12 hr of toxin exposure, returning to below baseline by 48 hr. The results indicate that lipocytes produce the M subunit of laminin in place of A. Production of this subunit by lipocytes may facilitate cell growth and reorganization during liver regeneration. (Hepatology 1994;19:764–770).