Exposure of Cephalosporium acremonium, strain 1435, to N-nitrozo-N-methylbiuret resulted in a changed variation coefficient with respect to two quantitative features--the antibiotic production and proteolytic activity. Correlation between the variation coefficient and mutagen exposure time was different for every feature. Positive correlation was found in variation with respect to the antibiotic production and proteolytic activity in populations of various Cephalosporium acremonium strains chosen with regard to one or two of the above features. The level and form of the correlation in variation of the above features in populations changed during selection. Selection according to the two quantitative features resulted in an increased correlation coefficient.