We compared the structure and subunit composition of laminin in less tumorigenic mouse A9 and highly tumorigenic mouse A9HT cells by pulse-chase studies. During a 15' pulse, the ratio of laminin B1 to B2 subunits in cell lysates is 1:1 in both the A9 and A9HT cells; however, after a 3 hr chase, this ratio changes to 6:1 and 2:1 in the A9 and A9HT cells, respectively. Analysis of mature laminin subunits in culture media after a 3 hr chase also showed a similar higher ratio of B1 to B2 in the A9 cells as compared to the A9HT cells. The higher ratio of B1 to B2 subunits in A9 cells was evident as early as after a 30' pulse. A comparative analysis of steady-state levels of mRNAs for the laminin subunits B1 and B2 between A9 and A9HT cells showed a ratio of 1:1 for B1 and a ratio of 1:1.65 for B2. The ratio of B1 to B2 mRNAs in A9 cells was 1:1.3 whereas in A9HT cells it was 1:2.5, suggesting changes in the processing of mRNA in the highly tumorigenic A9HT cells. These observations suggest that the processing of laminin B subunits is altered during the process of increased tumorigenicity, thus resulting in the synthesis and secretion of structurally different laminin in tumorigenic A9HT cells as compared to the parent and less tumorigenic A9 cells.