Hematoporphyrin derivative (Hpd) is a fluorescent dye that is preferentially incorporated by tissues with a high mitotic index, such as tumor cells and blast cells. A cytotoxic effect is produced following light activation. Previous studies have shown a long lasting reversible inhibition of DNA synthesis in Hpd-treated cells that failed to stimulate allogeneic lymphocytes in either primary or in secondary MLR. In this study we report Hpd inhibitory effects on some immunological assays in vitro. Treatment with Hpd of cytotoxic effector cells resulted in inhibition of their lytic activity likely dependent on the loss of binding to target cells. In the same way Hpd treatment inactivated the lytic activity of NK cells. In contrast Hpd-treatment of target cells did not modify the above immunological reactions. Moreover Con A agglutinability, antibody dependent capping as well as E-rosettes were inhibited following an Hpd treatment of relevant cells. Since normal susceptibility to humoral and cell mediated lysis was exhibited by Hpd-treated cells it is unlikely that cell surface molecules were damaged. An inhibitory effect exerted by an Hpd treatment on cell surface movements might explain these findings.