La formalina se ha utilizado como agente esterilizante, bactericida y germicida en clinicas y hospitales, sin estudios que soporten su uso o que garanticen los resultados esperados. El formaldehido, en su forma natural es un gas de bajo peso molecular, facilmente diluible en agua y de olor irritante caracteristico, este agente tiene gran tendencia a polimerizarse haciendo grupos de 3 y 4 moleculas obteniendose el Trioximetanal o Para – formaldehido (formalina). Se realizo un estudio con 86 piezas de instrumental odontologico utilizado en pacientes de la Clinica CES Sabaneta, el cual luego del lavado corriente, fue expuesto a la formalina por 12 horas, luego se hizo un frotis a cada instrumento. Se incubaron las muestras 12 horas y se hicieron repicas a las 36 y 48 horas obteniendo resultados negativos para crecimiento de microorganismos en todos los casos. El grupo control estuvo compuesto por 14 instrumentos a los que se les realizo el mismo procedimiento del grupo experimental, pero sin pasar por la formalina. La formalina fue 100% efectiva como agente bactericida en microorganismos aerobios. ABSTRACT The formalin has been used as a sterilizing, bactericidal and germicide agent in clinics and hospitals, without studies that support their use or that guarantee the expected results. The formaldehyde, in its natural form is a gas of low molecular weight, easily diluible in water and of characteristic irritating scent, this agent has great tendency to cure itself doing groups of 3 and 4 molecules obtaining themselves the trioximetanal or para - formaldehyde (formalin). This study was performed with used dental instruments in patients of Clinic CES Sabaneta, after the current washing, it was exposed to formalin means by 12 hours. 86 samples of laboratory were made, after the exposition of instruments to formalin during 12 hours; frotis to each one in all its surfaces were done to them. Samples were cultivated independently with number of sample and name of instrument. Cultures were incubated during 12 hours and retorts at .36 and 48 hours were done obtaining negative results for growth of microorganisms in all the cases. Control group was composed by 14 instruments in which the same procedure of the experimental group was made, but without the use of formalin. The formalin was 100% effective used as bactericidal agent in aerobic microorganisms. Key words: formalin, bactericidal, sterilization, dental instruments.