This study was run in both the presence and absence of metabolic activation. No mutagenic response was seen in four of the tester strains. However, in the fifth strain (TA-1535), a doubling of background mutation frequency was achieved, but only at the largest concentration tested (10 ul/plate). This response was not obtained in similar studies with high-purity TBA (99.9%). Moreover, this response is quite surprising, since the more sensitive tester strain for base-pair substitution (TA-100) showed no effect of G-TBA treatment whatsoever. This result is interpreted as a spurious one which either may not be repeatable or may have been due to a trace contaminant. The weak results seen in this study do not allow a conclusion that G-TBA is mutagenic in this assay system.