The cytotoxicity of methotrexate (MTX) on representative human tumour cell lines (two cell lines from head and neck carcinomas, two from breast carcinomas, two from osteosarcomas and one lymphoblastoid cell line) was evaluated to: (1) examine the optimal time interval between MTX and folinic acid (FA) administration; (2) determine the critical FA/MTX concentration ratios; and (3) compare the relative effects of the equimolar mixture d,I-FA and I-FA. The cytotoxic effects of MTX were assessed by the MTT semi-automated test. For all of the cell lines tested, a significant inverse relationship was noted between the degree of MTX cytotoxicity reversal and the duration of the time interval between MTX and FA administration. Overall an 18-24 h interval between MTX and FA represented a time-threshold after which MTX effects could not efficiently be reversed by FA in most cell lines. With shorter time intervals between MTX and FA, MTX cytotoxicity could be partially on even totally reversed by FA; the intensity of reversal varied among the cell lines tested, and depended on the FA/MTX ratio. Regardless of the interval between MTX and FA, analysis of the various FA/MTX ratios revealed a significant direct relationship between this ratio and the percentage of recovery. Presence of the d-form had no influence on the MTX rescue capacity of the I-form; this suggests that the presence of the d-FA is unlikely to have any significant clinical consequences.