When rat lymph node cells, rendered incapable of division by treatment with mitomycin C, were then reacted with sodium metaperiodate (NaIO4), they stimulated the transformation of untreated, syngeneic lymph node cells in vitro. Unequivocal evidence that untreated lymph node cells responded in this situation was obtained by means of sex chromosome karyotype analysis. In experiments in which the ratio of responder to stimulator cells was varied between the limits of pure responder and pure stimulator cells at a constant cell density, [3H]thymidine incorporation increased linearly to a maximum at a ratio of 1:1, and then decreased linearly. These results suggest a predominantly bicellular reaction in which one periodate-treated cell stimulates only one responder cell. In experiments in which stimulator cells were reacted with periodate, but not treated with mitomycin C, and then mixed at a 1:1 ratio with untreated, syngeneic lymph node cells, karyotype analysis showed that both periodate-treated and untreated lymph node cells responded in significant numbers. Lymph node cells were cultured at various cell densities under three different conditions: one group was made up entirely of cells treated with periodate; a second group consisted of a 1:1 mixture of cells treated with periodate and untreated cells; the third group contained normal cells cultured in the presence of soluble concanavalin A. Expressed as [3H]thymidine incorporated per 10(6) cells, a progressive increase was found at the lower cell densities for all three groups, and identical slopes (about 2.5) were obtained when the results were plotted on a full logarithmic scale. Mitomycin C-treated lymph node cells incubated with concanavalin A and then thoroughly washed were found to be capable of transforming untreated, syngeneic lymph node cells. Maximal [3H]thymidine incorporation also occurred at a ratio of 1:1. These results show that, similarly to periodate stimulation, mitogenic stimulation by concanavalin A probably operates through an indirect mechanism, analogous to a mixed lymphocyte reaction. It is possible that other mitogenic agents also operate indirectly.