Mice depleted of T cells by adult thymectomy, X-irradiation and reconstitution with syngeneic bone marrow cells, either untreated or treated with anti-Thy-1 serum and complement, were immunized intensively with alum-precipitated bovine serum albumin (AP-BSA) along with or without bacterial lipopolysaccharide (LPS), but no significant anti-BSA antibody response was detected. Priming of the T-cell depleted mice, however, either by a single injection of AP-BSA plus LPS or by multiple injections of AP-BSA without LPS, resulted in the generation of immunological memory. A single injection of AP-BSA without LPS was ineffective. The memory required the aid of syngeneic T cells to be recalled by the challenge with AP-BSA plus LPS. On the other hand, multiple injections of AP-BSA plus LPS did not cause the generation of memory and the response of these mice to the challenge was lower than that of unprimed control mice. These results suggest that (1) the anti-BSA response is highly dependent on the helper function of T cells, (2) the degree of T-cell requirement for the memory generation is very low, and (3) priming with too much strong stimulation in the absence of functional T cells leads to the suppression or abortion of previously generated immunological memory.