The blastogenic and mitogenic response of murine splenic lymphocytes to concanavalin A and the response of the resulting blasts to a lymphokine co-stimulator preparation was examined with an electronic particle counting and sizing apparatus (Coulter Counter and Channelizer). Mitogenesis, as determined by increase in cell number, correlated well with the time-integrated uptake of tritiated thymidine. It was found that co-stimulator was highly mitogenic to the lectin-induced blasts, and that the cells appearing in response to it were uniformly large (co-stimulator blasts). Furthermore, the eventual increase in cell number was proportional to the co-stimulator concentration, at least for the concentrations, cell numbers, and incubation times examined. This relatively simple assay that uses the Coulter Counter and Channelizer is therefore capable of substituting for serial measurements of tritiated thymidine uptake and yields additional information on the volume spectra of responding cells, as well as forming the basis for assaying co-stimulator activity. The co-stimulator blasts were found to be capable of killing a wide variety of allogeneic as well as syngeneic concanavalin A induced blasts. The level of killing in a 3-hr 51Cr release assay varied between 9% (for the syngeneic case) and 24% of the maximum obtained by water lysis.