When compared to controls (n = 30), human immunodeficiency virus type-1 (HIV-1)-positive individuals, either asymptomatic (n = 10) or diagnosed with acquired immunodeficiency syndrome (AIDS) (n = 10), showed a statistically significant decrease in the percentage and absolute number of CD4 ( + ) T-lymphocyte cells (flow cytometry, Becton Dickinson FACScan; mean +/- SD of 42.6 +/- 6.9 and 948.5 +/- 393.3, 19.5 +/- 8.7 and 269.8 +/- 174.3, 4.6 +/- 4.1 and 60.1 +/- 134.3, respectively; Student's t- test, p < 0.05). However, this decrease was less marked in asymptomatic patients; in fact, the percentage and number of the above cells in this group of subjects was significantly higher than in the AIDS patients (Student's t-test, p < 0.05). However, we failed to find significant differences in the percentage of natural killer cells (NKCs; CD15 ( + ) CD56 ( + ) ) between the HIV-1-infected asymptomatic or AIDS groups of patients, or when compared with the controls (mean +/- SD of 10.4% +/- 9.4%, 14.3% +/- 9.7%, and 14.8% +/- 6.4%, respectively). Whereas either group of patients had a lower number of NKCs per microliter than the control group (mean +/- SD of 137.8 +/- 87.6, 91.1 +/- 98.3, and 331. 5 +/- 266.5, respectively), this decrease only reached statistical significance for the AIDS patients (Student's t-test, p < 0.05). Healthy controls showed statistically significantly higher NKC activity than either the HIV-1-infected asymptomatic or AIDS group of patients (K-562 target cell; mean +/- SD and range values as percentage of specific lysis of 19.1% +/- 15.6% and 2.4%-58.2%, 3.4% +/- 3.2% and less than 0.1% [non-detectable]-10.3%, and 6.4% +/- 5. 5% and less than 0.1%-19.5%, respectively; Student's t-test, p < 0. 05). Challenge of samples from the control group with either interleukin-2, alpha-interferon, or with a mixture of the calcium ionophore A23187 (Io) plus the 12-O-tetradecanoylphorbol-13-acetate ester (TPA) resulted in every case in a statistically significant increase in NKC lytic function (mean +/- SD and range values as percentage of specific lysis of 19.1% +/- 15.6% and 2.4%-58.2%, 27. 6% +/- 17.4% and less than 0.1%-56.0%, 32.1% +/- 20.9% and 2.1%-76. 4%, and 62.6% +/- 24.0% and 16.7%-95.0%, respectively; Student's t-test, p < 0.05). A similar challenge for samples from the HIV-1-positive subjects, either asymptomatic or with AIDS, resulted in most cases in an enhanced NKC activity; however, this increase in NKC lytic function reached statistical significance only for the group of Io + TPA-incubated samples (Student's t-test, p < 0.05). These results indicate that control or patient baseline NKC activity, and the response of this cellular immune function to a challenge with different immunomodulators, are phenotype-independent. They also suggest an association between HIV-1 infection and alterations in the initial mechanisms responsible for NKC activation; a similar general explanation has been suggested to account for the abnormal NKC lytic function observed in various severe pathological conditions, e.g., extensive burns, polytrauma, and sepsis. Understanding the molecular mechanism involved in regulating initial NKC activation could provide the rational basis for the design of newer pharmacological strategies to treat these conditions.