To eliminate between-tests error in longitudinal studies, for specimen sharing, convenient scheduling, etc., it is necessary to freeze freshly separated leukocytes as well as non-transformed, continuous T lymphocyte (CTL) lines. To test the efficacy of a programmable freezer (temperature falls at an optimal rate), freshly isolated lymphocytes and CTLs were each aliquoted into three sets of vials. The cells were thawed and/or washed and assayed for viability and T helper (T(sub h)0/T suppressor (T(sub s)) ratio. It is clear that omadequate freezing non-randomly damages cells of T cell subpopulations, T(sub h) being more sensitive than T(sub s). Further, it is proved that inadequate cryopreservation can confound results from a number of assessment methods owing to morphological and functional damages. The Battery of Leukocyte Tests (BLT) under development in the laboratory is designed to detect toxic, immunotoxic and genotoxic effects of in vivo mutagens exposure to human blood.