Spleen cells exhibit a dose-dependent spontaneous proliferative response to administration of HA in vivo. Normal C57BL/6 mice were injected with increasing concentrations of HA or PBS (control) and 48 hours later, the spleens were harvested. The number of cells undergoing spontaneous proliferation were quantitated by direct addition of ³H-thymidine to the cultures and measuring the incorporation during the next 6 hours. Bars with an asterix denote statistically significant difference when compared with the control.