Development of total complement (C) and C1 activity was followed in Line 96 chickens from day 13 of embryonic life to 40 days post-hatching. Both activities were demonstrable on day 13, and levels rose slowly in the late prehatching period. At hatching, on day 21, there was a sharp rise in both activities; both titres were roughly five times those of day 19 embryos. Further increases were seen to about day 10, followed by a levelling off (perhaps even a drop in the case of C1) for about 10 days. On about the twenty-first day the titration curve rose again. The source of the C detected in the embryo and young chicken is unknown. The pattern is consistent with transfer from the egg, but it might also reflect synthesis by the developing animal.