The effects of contact with a solid surface (in the form of adhesive microcarrier beads (Cytodex)) on the induction of proliferation of human blood lymphocytes after exposure to concanavalin A (Con A) were investigated. The duration of the lag phase preceding DNA synthesis was identical in the presence and absence of beads. The kinetic course of initiation of DNA synthesis was exponential but with different rate constants both in the presence and in the absence of beads during the first 4 days of the culture period. Cytodex exerted its effect by increasing the proportion of cells initiating DNA synthesis in response to Con A. To elucidate whether lymphocytes responded to Con A exposure in a dose-dependent manner, cells were seeded at different concentrations and exposed to Con A for different times. At 36 and 48 h after start of stimulation the proportion of cells synthesizing DNA increased with increasing cell number in the cultures. In contrast, the portion of cells synthesizing DNA was markedly decreased at high cell densities 60, 72 and 96 h after start of stimulation. In the presence of microcarrier beads, however, the proportion of cells synthesizing DNA was proportional to the cell number in the cultures also after 60, 72 and 96 h.