Data on the aminoacyl-tRNA synthetase activities in normal liver and in three transplantable rat hepatomas, viz Yoshida's AH 130 and Morris's 5123C and 7793, were studied by means of factor analysis, a powerful technique of multivariate analysis particularly suitable foridentifying factors which theoretically may account for the pattern of interrelations between several variables. The analysis has been performed on 51 patterns of enzyme activities, covering 17 out of the 20 aminoacyl-tRNA synthetases; 80% on average of the total variability of the 17 enzyme activities may be accounted for by the first 4 factors extracted. The enzymes seem to fall into different groups, depending on their relationship with the factors thus identified. The results suggest that enzymes belonging to the same group share a common control mechanism, and are independent of the enzymes belonging to different groups, both in normal liver and in hepatomas.