The capacities of phagocytes of subpopulation P1 (F) and morula cells (MC) of holothurian Eupentacta fraudatrix to modulate apoptosis of each other as well as cytokine-dependent mechanisms and hormonal regulation of these cells’s interaction were studied. The 18-h treatment of F with supernatants, obtained after centrifugation of MC preincubated for 3 h with phosphate buffered saline (PSB) at the temperature of 22℃ (SMC3) resulted in a significant growth of apoptosis level. A 30-min incubation of F with supernatants of MC, preincubated for 24 h (SMC24), on the contrary, reduced the apoptosis level and increased the level of interleukine-1α (IL-1α)-like substances, and 24-h incubation did not influence apoptosis and reduced level of IL-1α-like substances. Thus, proapoptotic effects of MC’s supernatants in F inversely depended on time of their preincubation with PSB and directly on time of incubation with F. Additionally, this effect was opposite to variations in the level of IL-1α-like substances. The level of apoptosis declined after 30 min of incubation but elevated after 24 h at the inverse treatment of MC with supernatant, obtained after preincubation of F during 24 h (SF24). The level of IL-1α-like substances dropped after 30 min and insignificantly decreased after 24 h. Hence, SF24 proapoptotic effect directly depended on time of incubation with MC and did not correspond to variations in the level of IL-1α-like substances. 100 μM dexamethasone stimulated apoptosis in F and MC in an inverse time-dependent manner during 24-h preincubation, and supernatants of cell suspensions obtained after such preincubations, stimulated apoptosis and reduced the IL-1α-like substances level in target cells at both types of interaction. IL-1α-like substances are supposed to be mediators for MC’s effects in F, but not for F’s action on MC. In holothurians, steroid hormones apparently may participate in the regulation of the immune response and cell cooperation.