The sub-cellular and humoral concentrations of a collectin-like protein from the solitary tunicate, Styela plicata, were measured after in vivo challenge with the inflammatory agent, zymosan. Tunicates were injected with zymosan before hemocytes and serum were harvested, subjected to western blotting and immunostained with an anti-S. plicata collectin antibody to determine the relative titers of collectin-like proteins. Concentrations of the predominant 43 kDa collectin polypeptide were found to decrease in hemocytes immediately after zymosan injection, before rising to levels that were six times higher than controls within 96 h. Similarly, immunohistochemistry showed that the frequency of collectin-positive hemocytes in the circulating hemolymph increased significantly within 96 h of injection. Levels of the 43 kDa polypeptide in serum mirrored those of hemocytes. Humoral collectin concentrations decreased immediately after zymosan injection before rising, within 96 h post-injection, to levels three times higher than controls. This response to an inflammatory stimulus resembles that of mammalian collectins like mannose-binding lectin. The data suggest that, like its mammalian counterparts, the tunicate collectin acts as an acute phase antigen recognition protein.