The effect of a new antiulcer drug, solon (sofalcone), on gastric mucus viscosity, permeability to hydrogen ion and degradation by pepsin was investigated using an in vitro system. Preincubation of the isolated gastric mucus with solon produced a marked enhancement in mucus viscosity. This enhancement was concentration dependent and at 2.2 X 10(-1) M solon reached a value of 230%. Permeability measurements showed that 2.2 X 10(-2) M solon increased the retardation ability of mucus to hydrogen ion by 32%, while 43% increase was obtained with 2.2 X 10(-1) M solon. The drug had no effect on the viscosity and hydrogen ion retardation capacity of albumin. The results of peptic activity assay indicated that solon had an inhibitory effect on the rate of mucus and albumin proteolysis. The rate of inhibition of peptic activity was proportional to the solon concentration up to 1.0 X 10(-5) M, at which concentration the proteolysis of mucus was inhibited by 60% and that of albumin by 45%. The results obtained here under in vitro conditions suggest that solon, by inhibiting peptic erosion of the mucus layer, increasing its viscosity and enhancing the ability to impede the hydrogen ion penetration, strengthens the gastric mucosal integrity and thus could aid in ulcer healing.