Prior immunization of BALB/c mice with free light chains from myeloma protein 315 (L315) and its variable domain (VL315) inhibited the growth of subcutaneously injected MOPC315 tumour cells. The growth suppression observed after immunization with L315 was equivalent to that which resulted from immunization with the complete M315. VL315 and non-polymerized L315 did not elicit specific antibodies. Polymerized L315 induced both suppression of MOPC315 growth and antibodies specific for free L315; however, these antibodies did not react with the complete M315, nor were they absorbed by MOPC315 tumour cells. The data indicate that the suppression of tumour growth was mediated by specifically sensitized cells acting in the absence of antibodies against M315 or L315. Immunization with the variable domain of the heavy chain from M315 (VH315) had no effect on the growth of MOPC315. The M315 fragments and subunits that induced growth suppression were thus identical with those capable of inducing T helper cells in BALB/c mice.