Single cell suspensions of metrial gland tissue from rats at Day 14 of pregnancy were prepared for maintenance in vitro. During the first 2 days of culture IgG was detected in glycoprotein granule-containing granulated metrial gland (GMG) cells. Albumin was also detected in GMG cells at the same stages. The IgG and albumin were not detected during the next 4 days in culture. When metrial gland cells, maintained in vitro for 5 days, were incubated with rat serum for a further 24 h, IgG and albumin were detected in GMG cells. When similar cultures were incubated for 24 h with purified rat IgG or purified rat albumin, GMG cells were positive for IgG and albumin respectively. Albumin was not detected in GMG cells in wax sections of metrial gland tissue, although IgG has previously been demonstrated. The uptake of serum proteins by GMG cells in vitro has been clearly shown but the difference in IgG and albumin content of these cells in paraffin-wax sections indicates that the means by which IgG accumulates intracellularly may be different in vitro and in vivo.