Resistance to Eimeria acervulina was measured in two lines of chickens that had been divergently selected for high (H line) or low (L line) antibody (Ab) responses to SRBC, and in a randombred control (C) line originating from the same parental stock. Fecal oocyst output of cocks from the three lines from the 17th generation was estimated after primary and secondary infection with 2 x 105 oocysts. In addition, Ab responses to E. acervulina oocyst antigen and cellular immune responses in vitro to E. acervulina antigen were measured after primary and secondary infection with E. acervulina. No significant line differences were found with respect to fecal oocyst output after primary infection. Only at the end of the primary infection period, i.e., Day 15 postprimary infection, was a significantly lower fecal oocyst output found in the H line as compared to the C and L lines. After secondary infection, significantly higher fecal oocyst output was found in the C line. Significantly higher Ab response after primary and secondary infection were found in the H and C lines as compared to the L line. No line differences were found for cellular immune responses to E. acervulina oocyst antigen. These observations imply that selection for enhanced humoral immunity to SRBC did not result in enhanced resistance to E. acervulina in terms of fecal oocyst output. However, the H line might expel E. acervulina more rapidly than the other two lines. The absence of line differences in resistance to Eimeria is discussed with respect to the role of the humoral immune response.