Microcapsules (diameter range: 5 to 100 microns) prepared through interfacial cross-linking of proteins with terephthaloylchloride exhibited a cytotoxic effect on L 1210 cell cultures. IC50 was: 0.86 mg/ml +/- 0.24 for microcapsules prepared from human serum albumin (AT microcapsules) and 0.63 mg/ml +/- 0.05 for those obtained from egg white lysozyme (LT microcapsules). With K 562 cells IC50 were 0.42 +/- 0.11 mg/ml (AT microcapsules), 0.06 mg/ml (LT microcapsules). An increase in the cytotoxicity was observed when reducing the size of the microcapsules and when increasing the reaction pH or the terephthaloylchloride concentration, or the relative concentration of microcapsules vs cells. On the contrary, the cytotoxic effect decreased, when prolonging the cross-linking time. The activity was not affected when the microcapsules were washed with toluene or with an alkaline solute. The cytotoxic effect, which appears for relatively high doses, apparently involves a contact between the microcapsules and the cells and seems to be related with the degree of cross-linking of the constitutive protein.