The tail of bacteriophage λ was found to disassemble when treated in suspension with acid to a pH less than 2.8 or alkali to a pH greater than 11.8. Treatment of the tail of bacteriophage lambda results in the production of distinct subunit structures, discs when acid treated, and units unresolvable by electron microscopy when alkali treated. When returned to neutrality both subunits produce structures which resemble the original tail except that they can be up to several microns in length.The acid and alkali treatments produce structures that have high molecular weight. The acid-produced structure appears to be composed of six subunits arranged to form a disc with a central hole.The reactions were unaffected by temperature between 2 and 56 °C or by exposure to 8 M urea for 30 min. Ionic strengths greater than 0.7 M NaCl inhibited all the reactions as did prolonged exposure to urea at a concentration of 7 M or greater.The tail consists of at least two antigens as detected by the complement fixation test and serum-blocking test. While alkali treatment has no effect on either antigen, both acid and urea treatment at 7 M or greater were found to destroy the serum-blocking antigen.Results obtained suggested that the alkali dissociation reaction is initiated at the end which, in the complete phage, is attached to the head, and proceeds in a polar manner toward the bacteria-attaching tip. If the tail is treated with urea the reaction proceeds from either end. The acid reaction was unaffected by the tip barrier and therefore was not demonstrably polar.