The content of PDGF in human blastocyst culture medium (n = 8), serum (n = 12), and FF (n = 17) from natural IVF cycles was determined by an RIA specific for PDGF B-chain. The blastocysts were cultured under serum-free conditions throughout development. The findings show that PDGF B-chain is released into the culture medium of human blastocysts and that serum is positive, whereas FF is negative for PDGF.