Summary The murine teratocarcinoma OTT 6050, when injected intraperitoneally in strain 129Sv-Sl jCP mice, gives rise to Embryoid Bodies (EB) which, upon adhesion to plastic, differentiate in vitro into a variety of cell types. The mechanism of adhesion of EB to plastic culture flasks was studied under tissue culture conditions. A small proportion of EB taken from the peritoneal cavity rapidly adhere when fetal calf serum is present in the culture medium. Many if not all other EB eventually also adhere in the presence of serum, but at a very slow rate. It was shown that these EB, in the absence of serum, slowly acquire the ability to initiate rapid adhesion when serum is added. Both the acquisition of the ability to adhere and the process of serum-dependent rapid adhesion itself require metabolic activity of the EB.