The activation of human peripheral blood lymphocytes or isolated T lymphocytes by concanavalin A (Con A) is hightly potentiated by the presence of autologous, mitomycin C-treated monocytes. The optimal lymphocyte: monocyte ratio within a broad dose range is 1:1 when the incorporation of [14C]thymidine is expressed as total incorporation per culture tube and 1:10 when expressed per lymphocyte. A five-to-ten-fold increase of total DNA synthesis is noted in the presence of 10-90% monocytes. The data may help to explain the wide variations in Con A responsiveness of human peripheral lymphocytes which may be partly related to differences in purification which give rise to cell preparations containing varying amounts of monocytes.