Peripheral blood lymphocytes from cystic fibrosis (CF) patients with advanced diseases do not proliferate following exposure toPseudomonas aeruginosa(PA) antigens inin vitro.In this study, we sought to determine if CF lymphocyte unresponsiveness to PA is due to inhibitory factors present in CF plasma. Nineteen low-responder CF (LRCF) patients increased their mean lymphocyte proliferative response ([3H]thymidine incorporation) from 703 ± 133 to 3178 ± 811 net cpm when incubated in normal plasma. These increases do not reach the level of response seen in normal individuals (8510 ± 1323 net cpm). Ten of 19 patients did not increase their responses over 2000 net cpm. Plasma from LRCF patients does not inhibit responses of normal or homologous CF lymphocytes. Responses of normal individuals in autologous plasma were 7807 ± 1164 net cpm. The same lymphocytes incubated in 16 plasmas from LRCF patients gave responses of 7146 ± 1317 net cpm. Preincubation of the PA antigen in LRCF plasma increases rather than inhibits normal lymphocyte responses. LRCF plasma absorbed with PA no longer supports normal lymphocyte responses to PA. LRCF and normal plasma mixtures increase responses of normal lymphocytes to PA over responses in autologous plasma. Extensive preincubation and washing of LRCF lymphocytes to eliminate blocking immune complexes failed to restore the ability to respond to PA. These data suggest that the unresponsiveness to PA of lymphocytes from CF patients with advanced disease is due to alterations occurring at a cellular levelin vivo.This lymphocyte dysfunction cannot be reversed by normal plasmain vitro, nor can it be induced in normal lymphocytes by the use of CF plasma.SpeculationLymphocytes from cystic fibrosis (CF) patients with advanced disease fail to proliferate in response toPseudomonas aeruginosaantigens. This abnormality cannot be reversed by the use of normal plasma or by preincubation and washing of CF lymphocytesin vitro.Heat-inactivated CF plasma does not inhibit proliferation of normal lymphocytes in response toPseudomonas.These results indicate that changes in lymphocyte reactivity occurin vivoand are unresponsive to plasma substitutionin vitro.Further investigation of quantitative and functional aspects of lymphocyte and macrophage subpopulations may elucidate the nature and significance of the unresponsiveness of CF lymphocytes toPseudomonas aeruginosa.