This study contrasts the effect of chemotherapy-induced and viral-induced (HIV-1) immunocompromise on natural killer (NK) and lymphokine-activated killer (LAK) cell function. The ability of NK and LAK cells isolated from the peripheral blood of healthy controls, breast cancer patients receiving or not receiving adjuvant chemotherapy, and HIV-1 seropositive individuals to lyse K562 and U937 targets was determined. Exponential regression analysis of the cytolytic data was used to derive the cytolytic variables A (indicative of the maximal cytolytic kill of a target) and k (indicative of the lytic efficiency of individual effector cells). Overall LU20 values were ascertained and adjusted to incorporate absolute lymphocyte numbers. Such analysis indicates that the cytolytic NK and progenitor LAK cell pools are diminished in breast cancer patients receiving chemotherapy. However, the ability of individual NK and LAK cells from treated patients to lyse targets remain unchanged. In contrast, the diminution of NK and LAK cell function in HIV-1 seropositive individuals is associated with reductions in both NK and LAK cell pool sizes as well as their cytolytic functions.