A method is given for obtaining the approximate standard error of the anti-serum (or antigen) maxima titre in the complement fixation test when the maxima line is determined by only two points; the same formulae may be used to obtain a conservative estimate of the standard error when more than two points are available. If the degree of lysis in each reaction mixture can be scored more finely than into the three simple states of ‘no lysis’, ‘partial lysis’ and ‘complete lysis’, then more accurate estimates can be obtained. A method is also given for estimating the concentration of one antiserum (or antigen) relative to that of another when they react homologously.