Ritonavir increases cell viability of CD4⁺cells treated with Fas agonist (CH11). PBMC from normal donors were cultured with Fas agonist with and without ritonavir (5 nmol/L). Viability is expressed as percentage of untreated (ie, without Fas agonist) control. Cell viability was determined by PI staining. Examples of scattergrams are shown.