Bacillus thuringiensis, a Gram positive and spore-forming bacterium, produces d -endotoxins composed mainly by Cry and Cyt toxins which are powerful insecticidal proteins. They are packaged in a crystalline parasporal inclusion. Once crystals are ingested by the susceptible insect, they are solubilized under alkaline and reducing environments and activated by the midgut proteases of the insect larvae. The Cyt-type toxins have hemolytic (17) and cytolytic activity (3), and several reports have demonstrated that the in vivo and in vitro activation of them is essential for activity (1, 3, 5, 6, 7, 9). Several works have demostrated that Cyt-type toxins can be conjugated with antibodies, hormones and growth factors, in order to improve their native capacities to produce hemolytic and cytolytic effects over target cells like: mosquito cell lines, cancer cells (2) and erythrocytes of different animal species (17). B. thuringiensis subsp. medellin (serotype H30)(12) produces the Cyt1Ab1 toxin of 28 kDa, (10, 11), which has shown mosquitocidal activity against culicid larvae (10, 15). Additionally, it acts synergically with the 68 kDa crystal protein (10), and shares an 86% aminoacid identity with the Cyt1Aa1 toxin from B. thuringiensis subsp. israelensis (15). We analyzed the in vitro and in vivo activation of Cyt1Ab1 toxin with standard and midgut proteases. We also tested its hemolytic activity on sheep red blood cells and its mosquitocidal activity.