Disruption of the Schizosaccharomyces pombe ras1 gene results in a morphological transformation to large spheres, in contrast to wild-type cells which grow as rods. Chemical analysis of isolated cell walls showed no significant changes in saccharide content but an increase in protein and phosphate contents in ras1- walls relative to parent walls. Polymers tightly bound to the cell wall were solubilized by SDS treatment. Several compounds with molar mass ranging from 22 to 130 kDa and more were resolved by gel filtration and SDS-PAGE. Among low-molar-mass species, a component moving as a band at 31 kDa was conspicuous in ras1- cell walls. It was solubilized by heating in Tris-HCl buffer and shown to have a beta-1,3-glucanase activity against laminarin. The level of the enzyme was by 30% higher in the ras1- cell wall than in the wild-type cell wall. This enzyme may participate in the remodelling of the rigid glucan network and account (at least partially) for the aberrant cell shape. The ras1- cell wall contained a high level of charged polymers, especially phosphoproteins, raising the appealing possibility that ras1- is involved in a putative kinase cascade required to sense and respond to external stimuli destined for the cell wall. Although the present study shows that ras1 loss of function and altered cell wall composition are closely linked defects, it has still to be shown that the ras1 protein is directly involved in alterations found in the mutant cell walls.