[9] They hid with a Polish woman, Maria Sawicka, a member of Żegota, who before the war had run track with Regina,[10] and who had previously hidden Klepfisz when he had left the ghetto.

After receiving instruction from the Polish Home Army (AK) in making Molotov cocktails,[2] Klepfisz set up an underground bomb factory in the ghetto, while other members of the Jewish resistance smuggled in the necessary ingredients from the "Aryan side" (they had to be purchased from many unrelated suppliers so as not to raise suspicions).

[12] Though the extent of Klepfisz's operation is unknown, in 1964, Polish workmen carrying out work on the site of the former factory unearthed 100,000 explosive-filled glass detonators for Molotov cocktails.

[13] Together with Arie Wilner, Klepfisz also served as an intermediary between the Jewish Combat Organization and the Polish resistance movement organisation - Home Army.

[2] He was trained in explosives production by a Home Army soldier, Zbigniew Lewandowski (nom de guerre "Szyna"—"Rail").

[15] He was killed on the second day of the Uprising on Bonifraterska Street, in the brush-factory district,[10] protecting the retreat of his fellow soldiers (including Marek Edelman) by covering a German machine gun with his own body.