The Friend erythroleukemia cell line was used to study the and Riflcind, 1978), it has been shown that numerous agents, binding and bialogical properties of the photosensitizer he-of which the best known is DMSO (Friend et al., 1971), are matoporphyrin derivative (HPD) on a differentiating system. capable of converting FLC to a phenotype which mimics in In addition, the effect of cholesterol hemisuccinate (CHS) en-many respects normal, fully differentiated normoblasts (Malik richment Of cell membranes On HPD activity was tested On et al., 1979). It is therefore important to follow the effect of the Same cell system. Differentiation of Friend erythroleukeand resulted in a decreaed cell volume and an increased rate different stages of the differentiation pathway induced by apof hemoglobin synthesis as a function of the duration of DMSO propriate agents. In the present study, the binding capacity of treatment. Differentiated cells seem to bind less porphyrin HPD, and its biological effect on FLC at different stages of than their undifferentiated counterparts. Thus, cells treated differentiation induced with DMSO, were investigated. In adfor 6 days with DMSO bound 30-40% less dye than an identi-dition, the protective effect of exogenous cholesterol enrichcal number of untreated FLC. In contrast, a Similar inhibition ment of FLC membranes against the photodynamic activity of of both DNA and protein synthesis by photoactivated HPD HPD was studied. was evident in either DMSO-treated or untreated FLC. Enrichment of cell membranes with CHS led to the same degree of protection from the damaging activity of the photoactivated dye in both differentiated and undifferentiated FLC. The decreased binding of HPD to DMSO-treated FLC is most cell culture likely a result of a reduction in cell volume of differentiated A mouse erythroleukemic cell line 745 (Friend et al., 1971), cells and is not related to an intrinsic property of the differ-was in Dulbecco's modified ~~~l ~' medium (DMEM; GIBCO, Grand Island, NY) supplemented with 10% fetal calf entiation process.