The impact of high temperatures (33-38 degrees C) and high relative humidities (80-100%) on the applicability of TLC systems for drug identification was studied during a six month climatologic cycle in Jakarta, Indonesia. In general, the R(F) values as observed on the plates were substantially affected in comparison to values obtained at moderate climates: most substances gave higher R(F) values under the tropical conditions, although exceptions may occur as well. The deviations tended to increase with increasing humidities and could amount easily to 20-30 R(F) units. On the other hand, some TLC systems were more affected than others. Tropical conditions also had a negative effect on the reproducibility of the R(F) values. However, when an R(F) correction procedure was applied, using reference mixtures of standard drugs on each plate, accuracies as well as reproducibilities of the resulting R(F)(c) values were drastically improved and data thus corrected were found to be compatible with existing TLC data bases developed under moderate climatic conditions. These results are in line with earlier studies carried out in a relatively dry tropical climate. In the latter the observed R(F) values tended to be lower than the ones published in the literature, but the R(F) correction procedure was able to correct for this phenomenon.