Breeding resistance to stress and screening plant populations through callus culture and employment of somaclonal variations are important biotechnological tools. Response of 20 genotypes of safflower ( Carthamus tinctorius L.) to callus induction and in vitro salt tolerance with three different concentrations of NaCl (0, 100 and 200 mM) was evaluated. Evaluation of various explants of safflower to callus induction was also conducted. Analysis of genotypic responses to callus induction was conducted by calculating callus induction rate, callus diameter change (mm/day) and production of green spots in calli during four weeks incubation. In vitro salt tolerance of genotypes was assessed using callus diameter change (CDC), relative growth rate (RGR), necrosis rate, tolerance index (INTOL) and relative water content of callus (RWC) under salt stress conditions. Results of analysis of variance showed significant genotypic variation for callus induction. Genotypes ISF-14 and Mex.17-45 possessed the highest (95.5%) and the lowest (49.25%) callus induction rate, respectively. ISF-14 with the highest callus induction at first and second period, the highest callus production rate and mean callus induction of 95.5% was ranked as the best genotype for callus induction. On the other hand, Mex.17-45 with mean callus induction of 49.25% was ranked as the inferior genotype for callus induction. The tested genotypes showed significant differences for CDC ranging from 0.56 (Gila) to 0.36 (PI-301055) mm/day. Results of the explant responses to callus induction showed that the highest(72%) and the lowest (64%) callus induction rate were observed in hypocotyle and pedicle, respectively. Moreover, leaf and hypocotyle explants possessed the highest (0.38) and the lowest (0.28) callus diameter change, respectively. Results of in vitro salt tolerance assessment of the genotypes showed that salt stress caused significant reduction in RGR, CDC, RWC, INTOL and significant increase in necrosis rate. Genotype K 21 had the highest RWC, RGR and INTOL. The highest CDC and the lowest necrosis rate belonged to genotype Mex.6-97. Overall, among twenty genotypes, K 21 and Mex.6-97 were determined as the most salt tolerance at the cellular level. Genotype GE-62918 with the highest necrosis and the lowest of RGR and INTOL was introduced as the most salt sensitive at the cellular level. Condisering genetic variation for salt tolerance at the cellular level and utilization of water saving mechanism or cellular persistence in water deficiency conditions caused by salt stress, the use of salt-tolerant genotypes is recommended for future genetic and breeding studies. Keywords: response, safflower, stress, explant, tissue culture
