The synthesis of C1q by tissues of pig embryos during gestation was studied using specific incorporation of 14C-labeled amino acids into C1q in vitro. C1q synthessi was first demonstrated in intestinal tissue (on the 48th gestational day) and this was the most active site of synthesis of C1q at all times. Significant synthesis of C1q subsequently was demonstrated in lymph nodes and spleen of older embryos (by gestational days 66 to 79) and at still later stages in tissues from other highly lymphoid organs such as liver and lung (gestational day 90 and later). We propose that there is a special relationship between development of synthesis of C1q and development of the lymphoid tissues, which suggests that the lymphoid cell is the site of C1q synthesis.