Cultures of spleen cells from Swiss, C57Bl or DBA/2 mice stimulated with 2,4,6-trinitrophenyl (TNP) conjugated sheep erythrocytes (SRBC) were used for studying the in vitro responses to TNP and native SRBC antigens and the frequency of occurrence of cells responding to both antigens. The response was revealed by plating the cultured cells with both native SRBC and TNP-conjugated pigeon erythrocytes (TNP--PRBC). Specific responses were obtained in all the cultures. Bispecific haemolytic plaque-forming cells (PFC) were detected in almost all cultures of individual Swiss mice cells with a frequency of 2-5--14 PFC/10(6) cells recovered. In DBA/2 cell cultures bispecific PFC were found in half the cultures (2-5--8-3/10(6) cells) and in C57Bl cell cultures in 30 per cent of the cultures (7--21/10(6) cells). When cells from individual Swiss mice immunized in vivo with TNP--SRBC were as an allogeneic culture from the 2nd day after immunization in the presence of TNP--SRBC, the frequency of bispecific PFC increased from 8 to 30/10(6) cells. Mixed allogeneic cultures of normal C57Bl and DBA/2 cells yielded high specific responses with regular occurrence of bispecific PFC only when the numbers of cells cultured together was small. However, when allogeneic cells were mixed 24 hours after starting the cultures, all responses were stimulated and bispecific PFC were found in considerable numbers (4--33/10(6) cells). Cross-reactivity between TNP--PRBC and native SRBC antigens was studied by culturing cells with each of the antigens and plating the cells with both, or by immunizing in vivo with SRBC or PRBC and culturing the cells with both antigens from the 2nd to the 5th day after immunization with both antigens. In no instance did bispecific PFC exceed background levels (0-1--0-6/10(6) cells) in these control experiments.